Not hugely, but even at lower resolutions you still see gains from the GPU, just not as much as when you gme at higher resolutions.
Lower the res and the harder the CPU works. Thing is, GPU is more important then CPU, as i have a X2 4200+ Chip with a GTX465 and it still plays all the games out now pretty well @1600x XXX resolution (cant thing of it off hand)
But it all depends on the games you play and how much budget you have.
If its a tottally new build from sartch and monies a little tight, go the AMD route, as you get good preformance and at a good cost.