I had the GTS 640 before I got the 512 I have now, and it was a very solid card. With my system (in sig) it gave me 60+ fps on COD4, Oblivion, and Stalker, ~30 in Crysis on High settings no AA, ~40 on WiC, and I didn't really try the others.
The only time the 640 will have any advantage over the 320 is in higher resolutions, so if you're having trouble with some of them, try bumping down the resolution and see how it goes from there.
I thought that the 320mb wasn't quite enough for me at 1680x1050 in Oblivion and Quake Wars, but I like turning all my settings up. Overall I'd say 1680x1050 is right around the limit of the 320mb of memory, especially with AA turned on.
I have a XFX XXX 8800GTS 320mb, and besides crysis, everything i play can be maxed out. I'm also using an old amd fx-60 oc'd to 3ghz to push it, so i'm sure its being bottlenecked a bit. Oblivion, Bioshock, Witcher, Stalker, and COD4 all range from 30-60 fps, everything on high at 1650x1050. I have an Acer 22" btw. My gfx card is one of the overclocked out of the box editions, so your results may vary somewhat, but for the most part expect to be satisfied. In crysis i could run somethings on high and somethings on medium, but i think even the greatest system has to tweak a little bit on the settings to get the game to be playable at all times.
I used a 320MB GTS at 16x10 on an Acer 22" and before Crysis, Oblivion was the only game it struggled at some. Crysis however the card really struggled at that resolution. I ran some high and some medium too, but it got real slow in spots. You'll be happy if you don't expect to stay over 20 fps nor go higher than 16x10 medium in crysis.
I had an evga 8800 GTS 320mb before I stepped up to the GTS 512. I push the same 1680x1050.
It's a solid card that overclocks well. You should have no problem hitting 20-40 fps in crysis @ medium with shaders and effects at high. Everything else you can pretty much max out, tho WiC might get a bit choppy.
I have a BFG 8800GTS 320 OC 550/800. Since my LCD is just 17" so 320MB is all i need.
i OC it easily to 600 Mhz and mem to 920 mhz. The patch for Crysis and a 80-100 Mhz overclock on a stock GTS should give you a 6-9 FPS boost, wich is pretty good.