Although the G92 GTS isn't terribly better...

That was exactly what I'm doing right now, and thats why I got the GTS instead of the GT. The entire thing that we're banking on though is the fact if nVidia will release their 9 series by February or early March, while Step-up is still valid

The high-end 9 series will come out in Feb-March as Dragon said.
There will also be high-end ATI (r700) cards coming out about the same time.
They should be the first 8800GTX killers in a long time.

Just get the 8800GT or 3870 if you need a card now. Get the 8800GTS 512 if you are planning on keeping that a long time or using the Step-Up.
If you don't care about speed, a 2600/8600 is good enough,.

Actually, I have a 8800GT I don't know much about the Step-Up. Link please! Thanks!

http://www.evga.com/stepup/default.asp

Since everyone missed the point (sorry for the organization), I'll rephrase it.

Would it be stupid to step-up to the 8800GT instead of the GTS when I lose so much of my existing credit on the GTS 640?

UM Ya!!!!

That's what I thought. I didn't want to accidentally make a dumb decision, though.