To give you all an idea as to how the GeForce 7950 GX2's specifications compare to the previous flagship GeForce 7900 GTX and the 7900 GT, we've got a simple chart (courtesy of NVIDIA) that shows how the products differ.
As you can see, many of the GeForce 7950 GX2's specifications are doubled, like the pixel and vertex shaders and ROPs, because the GX2 features a pair of GPUs. Due to differences in clock speed, however, the GeForce 7950 GX2 doesn't have double the horsepower of a GeForce 7900 GTX, but it does have major advantages in a couple of categories, most notably pixel and texel fill rate and vertices per second.
NVIDIA PCI Express Inter-GPU Switching Topology
The GeForce 7950 GX2 has another feature not found on any of NVIDIA's single-GPU card as well. The GX2 is equipped with an NVIDIA designed PCI Express switch that allows each GPU to communicate over a single PCI Express x16 link. On the 7950GX2 the switch connects to the system over all 16 lanes, but to each GPU with 8 lanes. The reason NVIDIA went with 8 lanes of connectivity to each GPU, was to provide for shorter PCB board design while still delivering good performance. Turning off 16 lanes in the NVIDIA 48 lane PCIe switch also provides a bit of power savings. On the 7900 GX2, NVIDIA routes 16 lanes to each GPU from the switch, which is one of the reasons the 7900 GX2 is so much longer.
The two GPUs on the 7950 GX2 can talk through the switch peer-to-peer as well. The main reason for having this switch is for broad motherboard compatibility. The switch lets the GX2 work in motherboards with chipsets that don't permit splitting a 16 lane graphics connection into two 8 lane connections.
We should also note that the GX2 does not require a full PCI Express x16 electrical connection to function. When NVIDIA eventually supports Quad-SLI in the DIY segment, a pair of 7950 GX2s will function in older nForce 4 SLI based motherboards. Because the traffic coming in is already 8 lanes (from the chipset splitting a 16 lane PCIe connection into two 8 lane connections going to each PEG slot), the switch will interface with the 8 lane connection to the chipset, and just pass 8 lanes worth of traffic to each addressed GPU as needed. So, Quad-SLI will eventually be supported on all SLI capable chipsets like the NF5, NF4 SLIx16, and standard NF4 SLI.
Although the NVIDIA PCI Express switch is compliant with the standard PCI Express specification, the system BIOS of some motherboards may not properly recognize the 7950 GX2 right away. Without a properly configured system BIOS, the motherboard could fail to post or may not operate reliably. NVIDIA informed us that they have worked closely with motherboard manufacturers to have their system BIOSes updated to support the GX2. Please refer to this site for a list of known compatible motherboards and the proper BIOS versions that have been tested by NVIDIA. Please note the list on that site is not inclusive of all compatible motherboards, but rather reflects motherboards that have already been tested and qualified by NVIDIA. This list will be updated over time as more new motherboards are tested. And also notice the GX2 works on many platforms, including the 975X Express and CrossFire Xpress 3200.