I've decided to throw some hardware at my performance problem and it went away. I plugged my old GTX570 into the second PCIe slot, in addition to the GTX670 I already have. Dedicated the 570 to PhysX, and am seeing a dramatic performance increase in all my games, including DOS. I'm up to around 70fps on Ultra settings (everything turned on), from around 28fps on Low settings (everything turned off).
This doesn't make a whole lot of sense actually. I'm not sure if DOS uses PhysX, but even if it does, the fps increase I got seems a bit disproportionate. Oh well, can't complain, the game runs really smoothly now. Not to mention all the other games I'm able to max out now. Sweet.
