The video glitches you're describing could have a wide variety of causes:
1) Driver problems or outdated video drivers
2) Video card fan failure or insufficient cooling
3) Bad video memory on the video card
I'd try to diagnose them in that particular order. Start by uninstalling all of the video drivers you currently have installed for existing or old video cards. In particular, you don't want any Nvidia drivers installed. ATI drivers can generally overwrite older versions without a problem, but uninstalling first is probably a good idea.
Once you have the latest Catalyst drivers downloaded and installed, and assuming that doesn't fix things for you, try examining the video card itself. Is the heatsink fan spinning at full speed? Does the card seem to be getting enough outside air from the case fan? Do the spots not show up until the PC has been running for a little while? Does running the PC with the cover off seem to reduce or eliminate the problem.
If you are confident that you have a nice, clean installation of the latest drivers and that the card itself is running cool and you still have glitches, you probably need to start shopping for a new card. This assumes the graphical glitches show up all the time and not just with certain games or programs. Bad video memory on a graphic card will cause all kinds of havoc depending on where the bad memory is and how much is bad.
I have seen instances where the bad memory isn't often used unless you're running full motion video or playing 3D games and utilizing most of the card's resources. In these cases, the problem might seem application specific. The only way to really know for sure is to find a buddy with a 9800 Pro and try their card with the same programs and the same driver.
Good luck.