Well, my suggestion is to first decide how much you want to spend on things. The 8800GT models are about $160-210US versus about $220-270US. Once you figure that out, look at the specs for each of the ones in your price range and compare the following things:

Core clock 600MHz
Stream Processors 112
Memory Clock 1800MHz
Memory Size 512MB
Memory Interface 256-bit
Memory Type GDDR3

Any card that is lower than those is a cheap knock-off or mislabeled or underclocked version (usually due to them using cheaper capacitors/voltage regulators/etc.) and should be avoided. The rest all comes down to price and the amount of RAM (which, considering today's game offerings, isn't that big of a deal once you hit 512MB).

[edit] Doh! My apologies. That's the 8800GT model specs. Those still apply, but for the 8800GTS, here's what I'd use as a baseline:

GPU GeForce 8800GTS (G92)
Core clock 650MHz
Stream Processors 128
Memory Clock 1940MHz
Memory Size 512MB
Memory Interface 256-bit
Memory Type GDDR3