if you want to upgrade, get a gtx580 3g version maybe. and overclock your cpu to atleast 4ghz. even then you have to disable the ubersampling.

Asus P6x58D
Intel i7-930(3.8Mhz)
Asus GTX 570 DirectCU II(833Mhz)
6Gb Corsair DDR3
I got a GTX570 OC to GTX 580+ specs and only get an unplayable 30 FPS with Ultra!! SO I would say you would need to have a SLI configuration. It looked great on High so I'm not complaining cept Geralt's face and hands are washed out the other characters look OK.

Intel i7 950 (stock)
Gtx 460 sc SLi (stock)
12gb Ram (stock)

Nvidia 270 driver 1280 x 720 medium settings just about playable

Nvidia 275 beta drivers 1920 x 1080 ultra settings (include max texture and draw distance, no ubersampling) running like butter

strange but great...I lost the will to play this morning when I first install, shocking performance

If you disable SSAO, Anti aliasing and Ubersampling you should get quite a ways already. Those are the biggest GPU hogs.