heres a benchmark
that compares all the current gpu's (except the dx10 gpu's). If you want awesome grpaihcs, go for the 7900gt. But if your not a hardcore gamer and you want to save some money, then go with like a 7600gt.
Its up to you and how much money you want to spend, how long you want this gpu to last, and if you want max settings all the time
. But the 7900gt is the best card out of all the other cards your looking at, if thats what you really want to know.