SLI or Crossfire depends on whether you have ATi or nVidia cards.
Each has advantages and disadvantages, but neither has a clear lead over the other.
If it was me, I'd decide on the card and worry about dual card solutions later.
SLI on Wikipedia.Crossfire on Wikipedia.Hopefully those articles will explain more.
From what I can remember, prior to the X1950 series you needed a special Crossfire edition card for Crossfire. These are notoriously hard to find and can be expensive, so I'd recommend an X1950 series card if you go with ATi and you want Crossfire.
Edit: the articles explain the advantages and disadvantages of each compared to the other, including master cards etc.
So I'll leave it to Wikipedia to explain it.