3DMark 2006 is pretty even, there is no significant lead here. It is disappointing that QuadSLI gains only 2.5% over running single-card

techPowerUp! has posted a comparison between NVIDIA Quad-SLI vs. ATI Crossfire. Here's an excerpt:

When I first heard about the opportunity to play with QuadSLI vs. Crossfire I expected QuadSLI to have a significant advantage over Crossfire but at an equally significant price difference. Now I am very disappointed. The driver installation seems to be very unstable. Also it seems nobody tested these cards on an ASUS board, where you have to change a BIOS setting to get the cards stable. The performance is absolutely not what you would expect from a "technology leader", "best of CES" product. In some benchmarks two ATI video cards were a great deal faster, even at the high resolutions which NVIDIA says are the key benefits of QuadSLI.

Even more humiliating was that one single 7900 GX2 often runs faster than two GX2 in QuadSLI.

Dear NVIDIA,
I have a great performance improvement suggestion for your next QuadSLI driver release:
if ((resolutionX<1600) && (resolutionY<1200))
DisableQuadSLI();

If you already have a Ferrari and a nice boat and now need something new to impress your friends you could shell out at least 7k for a Quad SLI system and a 30" widescreen gaming display. All other users may want to go for a more cost-effective and stable solution.

Twice the power and twice the money don't bring twice the performance..surprise? :)

Update: Another Quad-SLI perfomance previews can be found on FiringSquad and Bit-Tech.