CPUs and RAM doesnt bottleneck a GPU...they don't directly communicate with each other like a CPU and the RAM does.
"But if you had PC3200 RAM and a better CPU it'd perform better" well duh, that's a given no matter what card or system you're using, it's not because it's bottlenecking a graphics card.
Run 3dmark05...that is basically all GPU power right there...chances are your score won't be ridiculously lower than say an AMD64 system using PC3200.....it'll be somewhat lower because of your RAM being PC2700....
right now your CPU is limiting you from running DDR400 speeds, so that's the first limitation right there IMO
all in all though I wouldn't really worry about it....why not just OC that CPU up to DDR400 speeds? Unless you don't have RAM to match then it'd be kind of pointless, but not only would your CPU be faster you could atleast run a 400 FSB system