In the last generation of graphics cards (eg ATI Radeon 9800, Nvidia Geforce 5950) ATI graphics cards are better than Nvidia. So, if you chose to get a older card like a ATI 9600 or 9800 or Nvidia 5600 or 5950 go with the ATI cards.
Current generation (ATI Radeon X800, Nvidia Geforce 6800) Nvidia cards are better than ATI cards. So out of the newer cards like Ati x600, x800 or Nvidia 6600, 6800 go with the Nvidia Cards.
Wayne