I've cross-posted this on another forum or two, but if it can save anyone my agony, its worth the annoyance of duplication.
Up until 5 mins ago I was tearing my hair out, getting ABYSMAL frame rates even at 800x600 HQ, this on an Opteron 150, 1GB Corsair XMS DDR 400, 6800GT, and a Raptor 72GB. I even reinstalled winblows XP. Nothing helped.
FInally, I saw the Tweak guide at techreport. The reviewer there had similar issues with an Nvidia card. He eventually disabled the secondary display (uncheck Extend my desktop on to this display).
I tried that, and now things are flying at 16x12!! wOOp. I'm going to have to start the whole frickin game over again (about 2 hours in) just to make up for the craptacular appearance/performance.
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