Another major difference was that NVIDIA launched the GeForce 7950 GX2 at retail. On the day of launch, enthusiasts could finally buy dual-GPU powered GeForce 79050 GX2 video cards from their favorite e-tailers. But, at launch, NVIDIA wasn’t quite ready to endorse Quad-SLI for the Do-It-Yourself market. Quad-SLI was only supported when purchased in a full system built by an approved partner. NVIDIA took this route to ensure Quad-SLI owners had a good experience and didn’t have to contend with incompatibilities or other potential pitfalls.
You can read the full article over at HotHardware, and have a second opinion at Guru3D. It seems that Quad-SLI is better than Dual-SLI by a nice little margin, but is it really going to be worth the extra cash?