Someone said they were getting choppy FR last nite, had them disable the "Z-Buffering" within AH in video config, solved their problem...also, I've got a similiar system to yours, I have the 32 meg card overclocked to 200 mhz and running 1024X res, with 32 bit colors, getting about 45-65 inflight.
Turn your Mhz up on monitor refresh (75 or more) if your monitor will allow it.
Make sure YOU manage your virtual memory, and not Windoze, by click the Advanced tab in under VIRTUAL from SYSYTEMS (Control panel) and double your MIN/MAX to whatever ram you have (ie. if you have 64, make the MIN/MAX 128)
Scandisk, defrag, reboot, cntl-alt-del and shut any unnecessary background applications before running the game.