It depends on what you play. I have a 6600gt... but it plays source at 1900x1200 so i dont need to upgrade because thats all i play.
Don't upgrade because you want somethin new (unless you have money to throw around of course); just wait till a game comes out
that you like but arent happy with the performance. Thats when you should upgrade, or wait till the next 'right' time to buy.