I personally like the EVGA brand. They often cost a little more, but I think they are worth it. Sometimes, NewEgg will have some recertified models for great prices. There's nothing wrong with them, and they are generally new cards, but for much better prices.
The following are the higher end cards:
GeForce 200 series:
GTX 260, GTX 275, GTX 285, G210, GTX 280, GT 220, GTX 295, GTS 250
...but the following are very good cards as well, and at a much better price:
GeForce 9 series:
9500 GS, 9400 GT, 9100, 9300, 9500 GT, 9300 GE, 9800 GT, 9300 SE, 9600 GT, 9800 GTX/GTX+, 9800 GX2, 9400, 9300 GS, 9600 GSO, 9600 GSO 512, 9200, 9600 GS
One thing you have to do is make sure your motherboard has the correct socket for the video card that you want to install, and that a socket is available, and that there is room for the card to be installed into that socket.
You can find some EVGA cards here, and they are a good representation of the cards available even if you should decide to go with some other brand and/or retailer:
Newegg.com - Computer Hardware,Video Cards & Video Devices,Desktop Graphics / Video Cards,EVGA