Did you add an extra 512MB of memory recently? (i.e. was the system fine until you went to 1GB of memory?). If so, go to ATI's website, there is an issue with the RADEON cards and more than 512MB of system memory.
If not, and I certainly hope this is not the case, your video card might be fried. My 9800 Pro went bonkers on me about 3 weeks after I got it. Ran great, then I started having all sorts of issues...one of them was very low frame rates just like you have. Are you also losing UDP a lot, or getting unexplained reboots or lockups? I also had that which initially had me thinking something other than the video card...but in the end, it was the RADEON all along. I returned my card to ATI via RMA and am awaiting the replacement (once I spent a week trying in vain to convince the ATI "Customer Care" people....who evidently don't speak English very well, that I had indeed performed all the little checks, updates, etc they kept telling me to do).
This may not be your issue at all, but just similar to what I recently experienced. If you have another video card, install it in your system and see what happens. Worse case, completely reload Windows from scratch and try that.