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
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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!