Games Fun. Graphics are great.
I have a Pent D 920, 2 gig ram, and 6800GS (475/1.1). The game auto detected me at High quality. I uped the res to 1280, and put on HDR.
In cities i get fps between 25 - 30. Dungeons ~50. Outside 15-25.
For the most part its playable however definitely not as smooth as games like HL etc. I did one quest in an "ice" themed area. There were intense bloom effects, transparency effects (ice), and my fps dropped to the high single digits.
This game kinda has me wanting to use evga's step-up and get a 7900 GT...ill have to decide in the next month. This is mainly because i want to play at highest quality @ 1280.
Verdict: 6800 GS plays the game, but you obviously won't be able to play at max settings. On high settings, most of the game is playable, but some areas (lots of foliage or special effects) get choppy.