I'm happy with the 4870 X2, which was the best card on the market at the time of my purchase.

Essentially, one card vs two cards in SLI/Crossfire...
A single card costs more, but is more powerful then either of the two cards. A single card allows you a second (also) more powerful card down the road for an upgrade if you choose to go that route.
Two cards allow more monitors if you want to use them, will cost less then one single stronger card, and if your card does go... your system can still run, rather then be out of commission.

Pretty much decide which factors are most important to you.
These were the factors I considered, but everyone has their own opinion.