HardOCP has taken a look together with id Software on the performance of Doom3 on current hardware, a good read but I mind you to keep this quote from John Carmack in your mind while reading it:

It should be noted that all of the modern cards play the game very well, and benchmark scores should not be the be-all-end-all decision maker. Scores will probably improve somewhat with future driver releases, and other factors like dual slots or dual power connectors can weigh against some of the high end cards.

Go read their article.