What foods comprised the staples of a Roman diet during this period. Would the aristocracy have access to significantly higher quality foods? Was geography a factor?
There was a great deal of social stratification and regional diversity in Ancient Rome, so your question has many answers. Kristina Killgrove did some interesting research on Roman skeletons from a bunch of different necropoles dating between the first and third century CE. By looking at carbon isotopes they were able to find out about the plants people ate, and nitrogen isotopes log information about protein sources. If you were poor in Rome, you were out of luck; you probably would have eaten a lot of millet, maybe some beans and vegetables (carrots, celery, broccoli, cabbage, lettuce, endive, onion, garlic, dates etc). You might occasionally get a scrap of pork. You would also be a locavore, so only local vegetables and fruits, and fish only if you lived in a coastal town, so geography was definitely a factor. However, a lot of cheap grain and olive oil was exported from North Africa and Egypt, so that's not to say food didn't travel.
Aristocrats could import exotic fruits, vegetables, and meats as they pleased; we hear a lot about extravagant feasts where stuffed dormice, quail eggs, snails, and fancy imported wines were served. Romans of all classes ate a fermented fish sauce called garum, they put that stuff on everything. Honey was the primary sweetener. And gladiators, believe it or not, were mostly vegetarian, subsisting on a diet of wheat, barley, beans, and a sports drink made from vinegar and plant ash.
At Pompeii they've found a lot of good information by going through toilets - yes, toilets, they're an amazing archaeological source. Being coastal, everyone ate fish and other seafood, and chicken eggs were apparently popular, with goose eggs reserved for the wealthy. They also found a very controversial giraffe leg bone - is it enough evidence to conclude that the Pompeiians ate giraffe? Probably not, but it's interesting to wonder how it got there.
Here's a link to some interesting Roman recipes: http://www.press.uchicago.edu/Misc/Chicago/233472.html
If anyone would like to chime in with more examples of how people ate in different times and places, please do.
Do u mean a citizen of the Roman empire or a citizen of the city of Rome? the empire itself was pretty vast so different people in different regions definitely had different staple diets.