Techreport has posted a review of nVIDIA's latest dual-GPU graphics card, the GeForce 7950 GX2, which also happens to be capable ("capable" being the key word) of Quad-SLI. You won't be surprised to read that this card is fast when compared to its predecessors. It positively destroyed all the other single-GPU cards the Techreport guys tested it against; in Battlefield 2 the GX2 managed "twice the average frame rate of the GeForce 7900 GT." As you probably already know, this kind of performance doesn't come cheap. NVIDIA expects the 7950 GX2 to cost around $599 to $649, and that's before you check your power bill: in tests the card drew 133 Watts at idle and a whopping 237 Watts under load. In comparison to the card's main single-GPU rival, ATI's X1900, the 7950 featured similar levels of power consumption, size and heat output but performed significantly faster in all the benchmarks. The 7950's dual-GPU solution also surpasses the performance of traditional SLI configurations like dual 7900 GTs, with the added advantage of being compatible with any PCI-e motherboard chipset.
Strangely, the biggest problem that the review found had nothing to do with the card itself. Although the 7950 GX2 is perfectly capable of being partnered up with another card to make a Quad-SLI system, nVIDIA refuses to support this type of configuration, citing the "complexity" involved. The only way you'll be able to get a Quad-SLI setup is by either hacking two cards together or by purchasing a (some say overpriced) system from Alienware, Falcon Northwest or Dell.
The company went as far as refusing to supply the website with a second review card. As the reviewer points out "when explaining to your best customers why they can't purchase two of your $649 video cards for themselves without also buying a $5K PC built by someone else, it's probably not good idea to use a shaky excuse with an embedded insult."