I agree with o1die regarding the MB.
Only once in my life I bought two cards for SLI and I was sorry in less than a month from the time of the purchase because it all came down to benching and posting screenshots on forums. After everyone applauded and the euphoria went down I had hard time selling the cards and exchanging two of them for one better.
2 graphic cards are more or less waste of money. Graphic cards wear off most easily out of the entire spectrum of hardware. And in your case plus paying for another mobo just seems not appropriate. I would keep the MOBO and buy 580 or 6970 (even better). Even better, I would go with i5/i7 system and I bet this will keep you happy a lot longer than SLI although you might be hardcore gamer who only cares about GPU horsepower above everything else.
On another hand, I went from Phenom II 720BE unlocked X4 and overclocked at 3.8 (with DDR2) to I7 2600K @ 5.0. I am happy by knowing that I have faster CPU, RAM and all that but besides the thoughts and the overwhelming feeling of having it, I do not see things moving noticeably faster with my GTX 460. But I am not that much of a gamer (not any more). Just spend your money wisely.