You can't roll back without using System Restore or the like. It's not officially supported. And yes, you generally need DX9 on there even with an older video card to play newer games (although it won't work properly unless your card supports the features the game needs). I really doubt DX9 is your problem.
I'm guessing, though, that since you went from an NVIDIA to an ATI card, that you didn't clean out the old NVIDIA drivers properly. Download Driver Cleaner 3, follow the included instructions, and you'll probably be working again...