I used to play Doom 3 at pretty high settings on my old 9800XT, 1280x1024 resolution. They're killer cards, but can still be a bit pricey (though they have come down some). The 9800 Pro's are a better value, if you decide to go that route. An unlocked, overclocked 6800 NU can run right with the 6800GT's and Ultra's, if you're not scared of playing around with the card (and not all cards will respond the same, so it's a bit of a gamble). If you don't mind spending a bit extra, go 6800GT. Close to Ultra performance, at reduced cost. I voted 6800GT.
Or I've got a few 4mb PCI bus video cards laying around here. If you wanna play big Doom and Wolf 3d how they're meant to be played.
I like turtles.