Check the CUDA cores on the 640 vs. the 650. If they are the same the PhysX performance should be very similar (depends on OEM versions vs. retail, Ti vs. non-Ti, etc.). Clock speed may play a small role, but the CUDA core count is most important. It may be that the 640 will do just fine if the CUDA core count matches up with the 650.http://1pcent.com/?p=169
Edit: After reading your post more carefully, it appears you are choosing between the 650 Ti (768 CUDA cores) vs. the 640 (384 CUDA cores). The 650 Ti, as shown in the link above is significantly better, but the question is whether its enough to justify the higher price. I'm guessing the answer is no, but you should also consider the 650 non-Ti for a cheaper alternative.