I used to have 512 megs of ram during the mod, when I added another stick to make 1024 I didn't notice any fps improvements to be honest, what I did notice was a litteraly halving of the load times (I've timed it) and windows picking up speed faster when you quit. with 512 megs you have to wait a minute or two for the system to 'recover' after quiting, but with 1024 there's no such problems.
Load times were acceptable with 512mb RAM, 3000+ athlon and with a GF4200 and I got a reasonable framerate, if that helps.