A used GTX compared to a new GTSv3 is still more power hungry, hotter, and slower at any resolution except 2560x1600. So the GTS wins.
But with a 640 GTS already you are only getting about 15% performance increase. Is it really worth 350$? Then again your current GTS is depreciating fast. I would wait 3 monthes for the GeForce 9. the G92 is an incomplete (DX10 vs DX10.1) mid range chip from the geforce 9 series. The new high end Geforce9 is right around the corner and we are guessing it will be a huge performance increase... It might even be twice as fast as your current GTS instead of 15% faster. So wait for it.
However if money is absolutely not an issue, then grab the new GTS to hold you up until the GF9 arrives.