All you should need are two equal speed PCI-E slots in which to put the cards. Motherboard makers are like every other company, and will try to sell you a more expensive product where the only real difference is they slapped the letters SLI onto the box, and raised the price like 15%.
I'm putting a new system together and I'm wondering about running 2 nvidia cards in SLI mode. Mainly in the area of motherboards actually, from what I've seen there are specific mobos for SLI/crossfire(ati specs), do you HAVE to buy one of these certain mobos or can you buy any mobo that has 2 pcie x 16 slots on it??