Like Kgury said, its dependent on your resolution. For something like 1680x1050, then a GTX 580 might be able to run it on Ultra just fine, but for 2560x1600, then you might need a beefier card.
According to these benches on Battlefield Bad Company 2 (which uses Frostbite 1.5 IIRC).
AnandTech - NVIDIA
It seems 2 GTX 580 SLI outperforms a GTX 590. However, I kind of doubt you need 2 GTX 580 SLI to max it out when the actual game comes out. Right now, they are still in Beta testing, so there is a chance the game will be optimized better and won't need that high of specs for Ultra. Also, a twitter update said the recommended settings will allow you to play on high.