The 7900GT usually outperforms 7800GTX because it has higher clock and memory speed
7900GS and 7800GTX are comparable, but 7900GS runs cooler and it is more overclockable. So, between 7900GS and 7800GTX, I choose 7900GS
By the way you can get 7950GT for cheaper price than 7800GTX
http://www.newegg.com/Product/Product.asp?Item=N82E16814143090
http://www.newegg.com/Product/Product.asp?Item=N82E16814170111
7900GS and 7800GTX are comparable, but 7900GS runs cooler and it is more overclockable. So, between 7900GS and 7800GTX, I choose 7900GS
By the way you can get 7950GT for cheaper price than 7800GTX
http://www.newegg.com/Product/Product.asp?Item=N82E16814143090
http://www.newegg.com/Product/Product.asp?Item=N82E16814170111