im like you man, gone with Nvidia forever, i tried ATI Radeon and their new cards, they are great , i found ATI has better graphics Quality , but speed performance Nvidia has ATI beat. im going to wait till Nvidia comes out with the 512mb DDR and 520mgz card,, till next upgrade, but for you i would go with ATI.
at least try it. if u dont like it 30days take it back.
remember always keep recipts