there had to be some that were truly gifted?

Certainly.

Amy Marcy Cheney Beach (1867-1944) was a composer and pianist. She was born into a very well doing family, a child prodigy who started composing at around 5 and started giving recitals at 8. She had formal musical instruction for a while, but was mostly self-taught, and became a professional musician for a few years. That is, until she married in 1885. She agreed to limit her performances and to donate the profits to some charities. Why? Because being a professional musician was not a proper thing for a married woman (as you mentioned, propriety and what not)... She almost exclusively focused on composition after that (something she could do while staying at home like a decent married woman). When her husband died, she went to give some concerts in Europe .

Here's her piano concerto. Not something you would imagine was composed, or played, by a rich stay at home wife.

Fanny Hensel née Mendelssohn Bartholdy (1805-1847) was also born into a prominent family. She, and her little brother Felix (yes, THE famous Felix Mendelssohn), received a musical education early on. She showed great ability, and contemporary musical super stars found her to be amazing. There had been women in her family who were active lovers and supporters of music, and some were also skilled players. Her father was not very supportive, and openly told her music could be her brother's profession, but for her it could only be a hobby. Felix thought publishing her music under her name would upset her womanly duties. She got married, and her husband was OK with having her music played at family concerts.

Here's a trio by her. I find it pretty damn good.

Anna Amalia of Prussia (1723 – 1787). She was the daughter of Frederick William I of Prussia and sister of Frederick II, it's hard to get any posher than that. Her father mistreated her and was completely against any formal musical instruction for her. She only managed to seriously study after his death. Her teacher was J. P. Kirnberger, who was a student of J. S. Bach. She became known as a patron of music, and a serious composer. She preserved a lot of music by Bach, Handel, Telemann, and others. She knew what she was preserving, it was apparently not just a hobby and she was not just collecting old music.

Here's a flute sonata. A nice work in the style of her time.

Maria Antonia Walpurgis Symphorosa (1724 – 1780). Her Serene Highness Maria Antonia, Princess of Bavaria, Duchess of Bavaria, was educated in the arts. She was a singer, harpsichordist and composer, also a patron of the arts with a refined taste. She studied under Porpora and Hasse (who were top notch musicians in their time). She composed a couple operas, writing both the music AND the libretto herself (not a very common thing to see).

Here's an opera composed by her. As Boromir would say, you don't just write an opera. It's a hell of a lot of work, and it's kind of a particular type of music. It doesn't matter if it ends being average, getting to compose "just another opera" is pretty solid evidence of musical skill.

Do we know of any upper-class women who were favorably compared to notable professional artists or musicians?

Maria Theresia von Paradis (1759 – 1824). She was the daughter of the Imperial Secretary of Commerce and Court Councilor to the Empress Maria Theresa. She lost her sight at an early age. In her teens, she was giving concerts, singing and playing the piano (and probably the organ). She apparently commissioned concert works by Salieri, Mozart and Haydn.

Here's some music was composed by her. Some times there is some confusion because there was this guy with a similar last name (Paradisi).

Bending a little the description you gave, we have Barbara Strozzi (1619 – 1677). She was probably an illegitimate daughter of Giulio Strozzi (his family was rival of the Medici, those guys were very rich). She studied under Francesco Cavalli (very respected and influential opera composer). After the death of her husband, she supported herself investing some money and making money from her compositions. She was a prolific composer, and published plenty of music. It was expensive to publish music back then, people were not doing it just for fun... You would need to be either loaded or have good chances of success with your music, so I guess she can be compared to notable professionals. She also sang and played the lute pretty well.

I love this recording of one of her songs.

Marianne von Martinez (1744 – 1812). I don't know if she was very upper-class, but her family got a patent of nobility. She had the most amazing musical education and contacts!

Metastasio (the top Italian librettist) was a friend of the family, and saw great potential in the little girl. So she took her under his wing. He got Haydn to giver her piano/harpsichord lessons. See, Joseph Haydn (who became one of the top musicians of the German speaking part of the world) and Nicola Porpora (who became an opera god) were her neighbours. Porpora gave her singing lessons, while Haydn accompanied her! Once she started composing, she took lessons from Hasse (super famous and super prolific opera composer). She met Charles Burney (a musicologist from those days, who managed to meet many interesting people). Apparently Mozart would visit the Martines some times, and he would probably play music for four-hands with her. Her music was known in many cities, but she got no official position (because that would be complete nonesense for a woman of her class).

Here's a harpsichord concerto by her.

Pardon the detour, but I guess I'll get to women born in less than super wealthy families.

Francesca Caccini (1587 – 1641) was the daughter of Giulio Caccini, one of the founders of opera itself. She was well educated (standard humanistic curriculum of the time: Latin, Greek, literature, maths) and also had musical training (which I guess would have been pretty solid, considering who her father was). She performed for kings and super wealthy people, and composed. She was working at courts with Jacopo Peri (another founder of opera). She was probably the first woman to compose an opera (apparently wrote quite a good number of them). She taught music to the Medici princesses.

Here's an opera composed by her.

Clara Schumann née Wieck (1819 – 1896) was one of the top pianists, male or female, of the 19th century. She had a very, VERY long concert career. About 60 years!

She was a child prodigy, and received a very solid music education. She married Robert Schumann, who became a very relevant pianist, composer, and critic of music (he studied with her father). Her husband spent quite some time in psychiatric hospitals, and he famously injured himself by using a stupid contraption to improve his piano technique (he was not able to be a concert pianist after that). She became the main source of income for the family. She and Robert had 8 children, so she would hire housekeepers and cooks to take care of the house while she was on concert tours. She apparently rescued her kids during a revolution in 1849. She also took care of some of her grandchildren when her daughter Julie died.

She stopped composing at a young age, not feeling very confident in her skills. In my opinion, ANYBODY who was in her social circle would feel very incompetent as a composer... She was married to Schumann and was a very good friend of Brahms. She met Schubert, Chopin, and Liszt. By the way, all of them considered her a top notch pianist, so I guess she was super incredibly amazing.

Here's her piano concerto. She was 16 when she composed it.

Guilhermina Augusta Xavier de Medim Suggia Carteado Mena (1885 – 1950). She was a top notch cellist, and became internationally famous (and kind of opened the possibility for women to become concert cellists in the 20th century). See, playing the cello was NOT something women were meant to do. It was considered a manly instrument, and the way the instrument has to be played...

Are you looking exclusively for married upper class women who took a hobby of music and became famous for it, or just if there are famous female musicians prior to the modern era?

Mary Casssatt was an outstanding female artist. However, there are some caveats to this suggestion. 1) She was born in Pennsylvania in 1844, moved to Paris in 1866 and spent most of her adult life living in western Europe. 2) Mary Cassatt never married or had children, although she did have a long term love-hate relationship with Edgar Degas. 3) She died in 1926, so she might be to recent for the time limits of your question.
Cassatt and Degas were leading members of the Impressionist Movement in late nineteenth century Paris and often held joint art shows at important art galleries. They were neighbors in Paris, but they argued about politics constantly. Edgar Degas was also a male chauvinist pig, who was famously quoted as saying "No woman has the right to draw like that", when he was commented on some of Mary Cassatt's paintings.
Mary Cassatt was a prolific artist. Indeed, she made it her profession and supported herself by selling her paintings. Her father was an upper middle class stock broker, but he would never send her any money, while she lived in Paris. Mary Cassatt's younger brother, Alexander, was the president of the Pennsylvania Railroad and was one of the wealthiest men in the United States, during the Gilded Age. Alexander would never directly subsidize his older sister, but did open the door for her to sell many paintings to the plutocrats in the United States. Many Mary Cassatt paintings have found their way into important art museums around the world. An important collection of her work was presented in the retrospective show "Mary Cassatt: Friends and Family" in 2008. Nancy Mowell Mathews was the curator of that show and wrote the companion coffee table book that has color illustrations of the paintings in that show. Nancy Mowell Mathews also wrote a good biography about the artist in 1994 "Mary Cassatt: A Life"