I guess in order to play this game without such horrid frame rates, I'm going to have to downgrade the version of driver for AMD. How do I downgrade?

I think you can find archives of previous drivers on AMD somewhere. Youd normally go to Control Panel and add/remove(or Programs) and launch the ATI one and do custom then select drivers. I have issues with the ATI tool not actually removing the drivers in Vista, so after uninstalling them that way Ill use Driver Sweeper to completely remove the left over drivers. Youre -supposed- to be able to just run the newer driver installer programs without first uninstalling old drivers, but from my experiences its not a good idea(as in, doesnt work).

So before you try a rollback to older drivers use Driver Sweeper and then reinstall 11.5+11.5a as that may work. If you do rollback I see a lot of people going to 11.2

1. Identify your graphic card. This is important because if you are using the 68xx series then you need to be careful how low you downgrade because some legacy drivers don't have support for this series.
2. Visit the link for Ati legacy drivers :

32 bit Windows : http://support.amd.com/us/gpudownload/windows/previous/Pages/radeonaiw_vista32.aspx
64 bit Windows : http://support.amd.com/us/gpudownload/windows/previous/Pages/radeonaiw_vista64.aspx

3. download the legacy driver you wish to test out but make sure to check the Release Notes for the legacy driver that you have picked to verify that your graphical card/series supports the driver. You will find the Release Notes on the top of every driver page.
4. Before installing the legacy driver you have to get rid of the old ones first. Go to Control Panel > Programs (Programs and Features) > Locate " ATI Catalyst Install Manager " > double click it and follow the Complete Uninstall feature. You will be asked to restart your computer when finished, if not do it anyway.
5. Now you are ready to install the legacy driver. Simply start the executable that you have downloaded and follow the instructions.

Note: Using software like Driver Sweeper can severely corrupt your system if you do not use it wisely and at the right moment. If you absolutely must use this software, never use it before Uninstalling your graphical drivers via the ATI Catalyst Install Manager. Once you have correctly uninstalled your drivers, then you may use the Driver Sweeper to look for any left overs.

I strongly recommend that you stay away from Driver Sweeper or similar software if you do not know what you are doing. Using the Catalyst Install Manager is sufficient enough to remove your drivers.