I just got the EVGA 8800 GTS 512. I really don't have any hands on experience with other current cards for comparison. I'm still testing this one as I just got my new computer set up.
However having just got through researching them and buying one...
I would not buy a factor overclocked card. Be careful they phrase overclocking differently to confuse you OCC, Superclocked etc. Compare clockspeeds on some cards (like different GT cards) and it looks like they OC and don't tell you anything at all sometimes.
The 2 top sellers of Nvida cards are XFX and EVGA the most difference discussed being in the warranties...
XFX has a double lifetime warranty meaning you can resell the card and the second owner will be under warranty (but not the third obviously)
EVGA has a step up program which means until 90 days past the purchase date you can exchange any regular card they make for another more expensive model and pay the difference
http://www.evga.com/stepup/default.asp?switch=2
The 8800 GTS 512 takes TWO slots up on your motherboard while the 8800 GT does not. Generally all the more powerful cards take two slots up to allow for the integrated cooling fan.
There are sometimes packaged deals on video cards. In the case of the GTS 512 there are some with multiple model numbers that indicate it's packaged with a game. The GTS without a game
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130325 and the one with Crysis
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130312. (note I got the without because the offer originally didn't give a price better than buying the game separately. Unfortunately the price dropped right after I bought one making the one with Crysis a much better deal -_-. Also the caption saying it's an overclocked card was added at the price drop and is incorrect according to EVGA itself (the caption should probably go on a different card they sell for 10 bucks more).
Wish I could give you more info on the GT but I've forgotten most of it and I got to go. Most sites seemed to say the GTS was better so I went with it. Of course if you cannot give up 2 slots or afford the difference the GT was my second choice.
Also the 9800s (the 9800 GTX is soon right???) are in the process of coming out so the possibility of a slight price drop in the next couple of months occurs to me.