During CeBIT, NVIDIA freed quad SLI from Dell's exclusivity. Using now 7900 clocked at 500/600, quad SLI is based on two double PCB cards. Each PCB includes one GeForce 7900 and 512 MB of GDDR3 memory.
As each 7900 GT includes two SLI ports, interconnections between GPUs are multiple. So if the two GPUs of a single card are interconnected in SLI, these GPU also connects to a GPU of the second card (GPU1 of card 1 with GPU1 of card 2, GPU 2 with GPU2). Each bi GPU card is equipped with a x48 PCI Express switch that is used as a bridge between the two GPU and the motherboard.
Of course, the question of the interest of Quad SLI remains. Indeed, the combination of the latest games plus one or two 30" monitors requires an enormous graphical power but the price of these monitors additionally to the Quad SLI (which hasn't been communicated but will probably be over 2000€ only for the cards) is incredibly high!
We will mainly recall the technological demonstration even if we still have to test this solution. It is important to point out that the 7900 Quad SLI will be restricted to the integration and will only be available in complete computers from a restricted number of partners selected by NVIDIA.