I dont think its ram either.. you wont even see a performance difference by adding ram, what more ram will do is stablize your fps not increase it.
you didnt even explain everything yet... what game is it stuttering in? and on what settings? and does this happen with all your games or just 1?
and i wouldnt jump to 1 conclution just yet with a problem with many variables. a driver confict would prolly cause stuttering. a hardware prob could cause stuttering. and running your games on settings above witch your hardware can support will cause stuttering.
if you dont have a temp monitoring program installed, you might wont to try one like 'Motherbourd monitor 5'. also just check and see what your CPU tem is in Bios b4 you bootup.
EDIT: and just incase, make shur its not somehting simple like forgetting to plug the extra molex power connector into your 6600gt