A GTS 640/320 is more like an 8800GS. (or the rebadged 9600GSO.) Both of these are below the 8800GT. The 8800GT, 4830, 9800GT, etc won't seem like much of an upgrade. These are faster, but not earth shattering so. The 4850/9800GTX is what you should be aiming for. How much you'll feel it depends on the rest of the system.