You've pretty much taken your system as far as it'll go....you might get a few more fps from a 9800pro.
That's why my old system with pretty much the same specs as yours is now my truckputer,lol.
Unfortunately,the fpu performance,front side bus speed,and an agp 4x are your limiting factors.
If you'd like to play your need for speed faster,I'd suggest going with a newer proc,mb,and mem.The nice thing is that you don't really have to have the absolute latest and greatest to accomplish your goals.
You could probably get where you want to go with say an athlon 64,a halfway decent motherboard with a nforce 4 or ati chipset,and 512 ddr400.
An Intel solution is technically slower by about 20%,depending on which benchmark you choose,but,I've always prefered them for stability.
That being said the nvidia 6800 or x800 are execellent video cards for the money.