EVGA 015-P3-1580-AR GeForce GTX 580 (Fermi) 1536MB
$459.99 after $20.00 Rebate Card $439.99 + $7.56 shipping.
Like mentioned in prior posts, single card will run all your games maxed out currently no problem on a single 1920x1080p monitor.
Two AMD 6950's are great performance I'd concur but comes with crossfire driver issues, heat issues & micro stutter. Again, still a lot of people do well regardless.
I'm a single GPU most powerful card solution choice type of person. It's been smooth and no problems.
Having said this, either choice you'll be happy with and will give you great performance.
Obviously the two AMD 6950's up front will surpass performance longer than one 580, but won't touch two 580's later down the road in hind sight by the time you actually need a second GTX 580. At which point you'd have saved yourself some time in headaches until absolutely needed.