May I suggest consideration of a 3rd alternative .......
Guru3D uses the following games in their test suite, COD-MW, Bad Company 2, Dirt 2, Far Cry 2, Metro 2033, Dawn of Discovery, Crysis Warhead. Total fps (summing fps in each game @ 1920 x 1200) for the various options are listed below:
In other words ......
.... the 580 will get ya 616 fps in Guru 3D's game test suite for $500 at a cost of 81 cents per frame
.... the 570 will get ya 524 fps in Guru 3D's game test suite for $340 at a cost of 65 cents per frame
.... the 560 will get ya 495 fps in Guru 3D's game test suite for $215 at a cost of 43 cents per frame
.... two 560s will get ya 862 fps in Guru 3D's game test suite for $430 at a cost of 50 cents per frame
.... two 570s will get ya 873 fps in Guru 3D's game test suite for $680 at a cost of 78 cents per frame
.... two 580s will get ya 953 fps in Guru 3D's game test suite for $1000 at a cost of 1.05 cents per frame
So ....
-two 560's get ya 40% more fps than a single 580 for as much as $70 less
-two 580's get ya just 10% more than the the two 560's for $570 more
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007709%20600030348%20600094002%20600107147&IsNodeId=1&bop=And&SrchInDesc=900Mhz&Page=1&PageSize=20
In years past, I'd have gotten the single high powered card to leave room for the upgrade later...... But to spend $1,000 versus $430 to get 10% more fps .... some time down the line ? I'd rather get the 40% boost from the getgo......and great 850 watt PSU's (W/ 10.0 jonnyguru performance ratings) can be had from as little as $115