I'm afraid it is your system, you are running nearly twice as much computer as me (except processor) and I never go below 34 FPS no matter what.
You have more system memoryl... I'm not sure how your card compares with the FX5700 128meg, which I am running, but I would assume yours is superior.
Are you running apps in the sysTray? Messenger, various unneeded silly print monitors etc?
Something is most assuredly wrong with that machine if it can't push decent frames with the sliders set as you are describing. My ground detail is at 3 miles, and all the other sliders are at about 2/3rds full detail.
I run 1600x1200.
Do some diags, something is not right in that thing.