yeah, but look at any benchmark tests of actual games, and you will see different for 'most' games, although not all.
there are a few that perform better with ati card, but most seem to perform better with the gtx580.
some benchmark links just googled,
http://hardforum.com/showthread.php?t=1568979
http://www.techspot.com/review/337-asus-geforce-gtx-580/page6.html
http://www.hwcompare.com/9289/geforce-gtx-580-3gb-vs-radeon-hd-6970/comment-page-1/
there both good cards, but i know for sure which i'd get if moneys no option.