On more recent games your cpu will be the bottleneck yes. The 4850 will not be able to show its full force, however, you should see a definate increase in performance. Its quite hard to say how much of an increase it will be because its difficult to say how much it will bottleneck, and it also depends on what games you play (ie alot of new ones have mulicore support).
Still, judging from your current 6600GT, i would think you would get at least double the performance on some games if not more - again it will depend on the game.