Hate to say what you're probably already thinking but your best option is to do a total reload. I feel your pain but if you want to be totally sure just do it. Look for a post created by myself on this forum and you'll see a history of problems I had with my Radeon 9800. Tried all the tricks involved in going from Nvidia to ATI and still had freezes. Finally, I did a format/install of XP Pro and (fingers crossed) so far I've had no problems/freezes. I'm not positive it's stable yet but I'm about 3 hours of patches/updates, proggie installs, and some web browsing. No way that happened before the reload.
NOTE: This is done with default settings. No tweaks with BIOS settings or SmartGART. I'm headed to bed soon and I hope I don't wake to a blank screen, frozen screen, or BSOD. 😉
Good Luck! 🙂