Could be many things causing what you describe, and it could be merely a coincidence that it happened after installing the video card. Just because that was the last major change you made before noticing the problem, dont focus on it entirely or it could make you overlook something simple. Best way to figure it out is to send Skuzzy a copy of your DxDiagnostic report, or post it here. I would definitely check the drivers, make sure all old ones were properly uninstalled, etc.
One of the toughest things going from an Nvidia card to an ATI card if you have a MOBO with the Nvidia chipset is making sure you completely uninstall the Nvidia card drivers without messing up the MOBO drivers. Best way I have found to do it is to uninstall EVERYTHING Nvidia, and then after installing the ATI video card, install your drivers for your Nvidia chipset on your MOBO. Also, make sure if you are using a separate sound card that you have the onboard sound disabled so there are no conflicts and its not eating up resources. Dont forget to make sure you have the latest drivers from Nvidia for your Nforce board.
Once you have done all that, reinstall DX9 to make sure everything is where it should be and is looking for the things it should be looking for and in the right places (i.e. make sure its not confused and still looking for the Nvidia card). This sounds silly, but I've had it happen more than once where an uninstall and new install werent picked up by Direct X. Run the Diagnostic utility and run all the tests. Make sure your video and sound pass all the DX tests.
Now that the system is cleaned up and ready to go, go into your AHII folder and delete the video8.cfg file to make sure it doesnt have any old settings from the last card. Start the game, setup your video, and you should be good to go.
If it still doesnt work, shoot Skuzzy a copy of your new DxDiagnostic and he can help you with any problems on the new setup.
Good Luck!