only buy from newegg or zipzoomfly, they're the best.... they have very good service, usually theyre cheaper than everyone else (a few exceptions are there, and for a GTO2?), and zipzoomfly is CNET certified....
i might add they got quite a selection....
as for your card, well... I would say get a 256MB card, 128 is definitly not enough, and 512 is overkill.... as for which model, well...
6600 GT is a great deal, but you can only get about 128 MB on the card if thats what ur gunna get. you may be able to go with an X700 Vanilla, its pretty quick, its pretty close to the 6600 GT, and its only 130 for the AGP version, but its all up to you dude...