This from Xbit and is a bit dated:
The new GeForce 8800 GTS 320MB is a good buy for its official price of $299. Among its advantages are support of DirectX 10, high-quality anisotropic filtering and good performance in common display resolutions. So, if you are going to play in resolutions of 1280x1024 or 1600x1200, the GeForce 8800 GTS 320MB will be an excellent choice.
Unfortunately, the technically promising graphics card, which differs from the more expensive version in the amount of memory only, is sometimes much slower than the GeForce 8800 GTS 640MB not only in games that demand a lot of graphics memory (e.g. Serious Sam 2 ), but also in applications that didn’t reveal any difference between graphics cards with 512MB and 256MB of memory before. Particularly, these are TES IV: Oblivion, Neverwinter Nights 2, and F.E.A.R. Extraction Point . 320MB is considerably more than 256MB, so this is a memory management problem, probably a driver issue. Anyway, even with the mentioned drawbacks, the GeForce 8800 GTS 320MB looks much more appealing than the GeForce 7950 GT or Radeon X1950 XT, even though the latter are going to become cheaper now.
Of course now there are much newer drivers available. I think the similiarly clocked cards are very comparable in performance until the need for the additional memory comes into play. I have the 640mb version but in short, go with the 320mb unless you play a game that requires the addtional memory or use a display greater than 1600x1200.