Well, you need a two PCI-e slots. That is a must, I think.
A motherboard with a SLi chipset is best, a regular nVidia chipset is fine, an ATI chipset is flaky. There are mods to make boards with 2 PCI-e, but no SLi Chipset, into full SLi motherboards. Requires a bit of tweaking.
Need a SLi-Ready card, and not just for the SLi connection. Think it has an extra chip inside it, not sure.
SLi is mostly software, though. Just need the slots. You incur a small performance hit without the connector.