Yes ATI is generally giving you more power for your money right now. Good thing you are not planning to SLI your ATI card as you cannot.
(It is called crossfire with ATI cards which your board does not support as you know)
Before plugging in your new card, uninstall your nvidia drivers. Go into safe mode and run driver sweeper.
Guru3D - Driver Sweeper (Setup) download from Guru3D.com
Shut down and install your new card then install the latest drivers from ATI's website for your particular card and OS.