Funnily enough, toms disagrees. I'll quote...
Any Radeon HD 6950 CrossFire solution beats any GeForce GTX 570 SLI configuration at 2560x1600, regardless of whether two or three cards are used. This is probably the only resolution in today’s test that really matters, since a single card will pass most tests at anything less.
Things get ugly for Nvidia at 2560x1600. While a 67% performance gain for two-way and 112% gain for three-way SLI certainly aren't shabby, AMD’s 92% two-way and 145% 3-way gains put it in the winner’s circle.
This, in spite of the fact that one of our games, Aliens vs. Predator, didn’t even benefit from the third AMD card.
Let's have a look at efficiency...
Oh dear that doesn't look good. Final quote?
CrossFire came out with a huge overall scaling lead over SLI, and removing the one title that didn’t reflect that average would have made the lead even bigger. Superior scaling allowed two mid-priced Radeon HD 6950s to approximate the performance of two higher-cost GeForce GTX 570s, while three HD 6950s took the performance win over three GTX 570s.
I'll keep it simple. You'd have to be brain dead to think Sli 570's were even a consideration vs Crossfire 6950's.