Hi,
I'm new to this forum, but I think I maye have the solution for you.
I have a ATI Radeon x800 Pro 256mb, on a MSI K8N Neo Platinum, 3500+
I was running into problems with Call of Duty and United Offensive only. Reinstalling the newest drivers for ATI never helped me, but I was able to get a hold on the Catalyst 4.9 instead of the 4.10, and it works great.
I have no problems anymore, and trust me, it would lock up my computer so the only way to get back was to acutally reset the machine, now it doesn't.
So, if you get the Catalyst 4.9, you should be OK to play. I'm going to attempt the 4.11 that has been released only recently. If I find out more about it, I will let people know.
I hope this helps you.
PEACE