25FPS avg? I get around 32-35Avg. try Overclocking the CPU some more
I also run at 1680x1050
GO with SLI those cards are dirt cheap now and your FPS will climb
Honestly I would sell your GTS 640mb and upgrade to either an 8800GTS G92 or 9800GTX. Even the 8800GT would be quite a jump.
The 8800GT is the best bang for buck, with the 8800GTS G92 following in a close second, but I picked the 9800GTX because it has a "9" in front so that means it is better.
Actually I picked it because they seem to OC better, but some 8800GTS G92 owners debate this regardless of the numbers floating around.