The 7970 outperforms both a 580 and 560sli, has higher memory bit rate, 3 gigs of ram.
just go back and check previous generations at which point did a new high end card outperformed 2 high end cards of the previous generation????
edit: thought you said 2 580, two 560 wont be far behind a 7970 I think.
selling 2 560s and adding some money to it to get a 7970 doesn't worth it IMO.