Some tips to improve performance:
Don't set General Draw Quality over 8 (There is little to no quality improvement, and none at all if you use any AA)
Don't use Ambient Occlusion
Play with your AA settings (Some cards seem to have a preference where 2x or 4x AA actually performs better than none)
Play with shadows (shadows are the easiest thing for the brain to gloss over because you rarely if ever focus on them directly)
Force vertical sync in your catalyst/nVidia control panel
Run Full Screen Mode (this will increase performance noticeably unless you have a REALLY good system)

Best way to improve performance is buy a new system / upgrade, but of course that costs money.