Are you saying ATI has better graphics performance in general, or are you referring to the SLI/Crossfire setup?SLI/Crossfire is a great marketing tool, but the bang for you buck is minimal. Besides, the human eye can't see any faster movement than 30 FPS, so why do we need 100 FPS? We don't. The only reason I'd go SLI or Crossfire is better looking graphics, of which ATI does better in that arena. But still, in a few months, that money would be wasted as a new single chip card will out-do any dual card that may be purchased at the time. And cheaper dual card setups are usually not any faster or better looking graphically than a single card at twice the price of one cheaper card.
Basically, unless you're bleeding edge and got money to burn, it's not worth it.