get the 6600gt, there's really no point (imo) to get something like the 6800gt b/c the 6600 and the 6800 will pretty much phase out and 6600gt is probably enough to run whatever games you want; around the same time, and by then, you'll have around $200 more to spend on a newer card
however, the x800xl is also a good choice if u got a lil more money
however, the x800xl is also a good choice if u got a lil more money