Here are some recommendations:
First, you went from an Nvidia card to an ATI.
You probably went to add remove programs, removed nvidia drivers, then downloaded and installed latest ati drivers. That would not be bad but as you have issues try this:
http://www.guru3d.com/category/driversweeper/
Clean out entirely all GPU drivers, left over dll files and registry... it might help.
Next, as you know that your cpu is bottlenecking the videocard, you might as well move up your bus from 200 to 210... that should give you an extra 150Mhz on your quad... might help a tiny bit and will not heat up much more.
Graphic settings:
I recommend native monitor resolution.
Everything on ultra with the exception of shadows on either low or high.
No AA
No Bloom, put in blur if you want.
This looks good and performs really well on my system that isn't all that different from yours... besides a lower end videocard. (GTX260)
If you still aren't pushing between minimum 45 to 80+ fps, just for testing sake, try setting the sound to 16 instead of 32.
Unfortunately all sounds go through CPU and not audio card, so that might just be it. (the extra stress due to graphics card might just be overwhelming on the cpu)
All in all, I am hoping the game will be perfectly optimized within the next few months for everyone.
Best of luck and study hard