8800 GTX is MUCH better, but it's EXPENSIVE (well, duh) and your gonna need a new motherboard if you have agp. And about the CPUs, if you wanna overclock, Intel Core 2 Duo, if not,go with AMD Athlon 64 X2.
Oh, and for a graphics card, either go for an ATi Radeon x1650pro or GeForce 7600gt
