I don't mean any offense but, once the company that makes the card has dropped support, don't you think it's time to upgrade?
I've got a radeon 4850. Not exactly top of the line
but it runs this game beautifully on almost all high settings. I'm actually pretty shocked that it does. Most games that look this good I'd be stuck at medium, maybe a couple high. (For an extreme example, I DLd Cryostasis while waiting for Amnesia, and that thing just will not run at anywhere near a reasonable resolution).
Granted, that game is wicked poorly optimized. This game is the opposite!