Step-up yes, normal upgrade, no.
I agree... granted the 8800GT and 8800GTS(G92) are great cards for the price... they are only slightly faster than what you have right now. You would be much better off waiting until late next year when the newer gen cards come out and investing in them.
I would start working on an overclock and see what you can get your current card to do right now.
The Tom's review gave the GTS 512MB 30% more performance than the GTS 640. That isn't slightly faster.