Theres is nothing between the 6800GT/GS and 7800GT because thats all they made
6800GS, 6800GT, 6800ultra, 7800GT, 7800GTX, 7800GTX 512MB
None of ATI cards are to look at because yuo would be looking at their X8** series which absolutely sucked. Their X1900XT cards are good but 500 dollars looks to pricey in your budget.
For the best card would defintely be the 7800GT at the 300 range, and the X1900XT at the 500 dollar range.