I am an ambitious white collar professional entering the workforce in USSR in mid-1950's. How successful can I be without joining the Party? What differences would be there as compared to some one who is a Party member?

You might consider checking out the book The Whisperers by Orlando Figes for some primary source material that will touch on the experiences of workers inside and outside of the party. I'm not sure what "ambitious" means to you in the context of the USSR in the 1950s. Success was defined very differently in the communist USSR than it was in the capitalist USA.