SLI and Crossfire require a capable motherboard, SLI can use any two of the same cards to display the screen and the cards are connected via a small pcb board that link the two.
Crossfire requires a Crossfire Mobo (duh!) BUT requires a "master" card and second card of the same model. The cards are linked outside the computer with dongles(cables) unlike SLI.
Nvidia has one up on ATi though with the 7950GX2 which is TWO 7900GTX's on top of each other for a total of 1GB (512MB effective) in a single slot. So you can have two 7950GX2's and run Quad SLI. Pretty nefty for extreme high resolution gaming.
I have a single 7950Gx2 and it pretty much eats up anything I throw at it (1680x1050res).
You can go AMD or Intel on either platform.