x1900xtx.
and i have the 7900gtx. ATI for the win with avivo video converter and compatibility for hdr + fsaa simultaneously.
if you play oblivion the x1900xtx will definitely be better because then you can set fsaa and hdr on all at once, and trust me you'll want both if you pay that much for a card.
but, performance wise the 7900gtx is slightly better in some games, and the x1900xtx is slightly better than others. they're fairly well matched cards though, definitely great for gaming.
whatever you do get 512mb of video memory, definitely will help in more memory intensive games.