Bidybag :
The GT only has 512MB and the GTS has 640MB. I thought this was the most important thing so the extra MB would make up for the slower clock speeds.
And this is not true at all. Memory size, although it does affect performance, really only matters depending on the resolution you play at.
So if you play at a really small resolution, like 800x600, it wouldn't really matter if you had 256MB or 512MB, because the card wouldn't use all of that memory. But as the resolution increases, memory does become more relevant.
And this is only for today's games. I'm sure in ten years or whatever you're going to need like at least 1 gig of onboard memory just to play at 800x600, because the textures and other aspects of the game will be so complex.
So there are a few things that can affect the amount of video memory you need, but generally in the short term, you just need to consider the resolution you play at. I would suggest 512MB or more if you play at 1680x1050 or 1600x1200 or higher. And I think by now with today's games, and with prices so cheap, I wouldn't suggest anyone get a card with 256MB unless they are playing at less than 1280x1024.
So your 640MB should be fine for quite some time. And I do realize that isn't what you asked at all, but sometimes I go off on tangents....
And yes, the 8800gt is faster, but not by much, and as others have said, it would be better just to keep what you have and wait until something comes out that will truly be a nice upgrade.