PCI Express and SLI
It's no secret that PCI Express is the bus interconnect of the future - Intel has made that much clear by now. Graphics card manufacturers have already stated that they'd continue selling AGP technology, but with the caveat that slowly but surely, PCI Express would take over. No matter the time frame, that a painful statement to hear if you're looking for a brand new gaming card. The natural inclination is to find a motherboard with PCI Express support as quickly as possible to avoid investing in a dying interface.
There's only one problem: until now, Intel had the only product in town that supported PCI Express. So, if you wanted to go PCI Express, it involved a new motherboard, processor, graphics card, and memory, if you opted for DDR2. Fortunately, NVIDIA's changing the rules with nForce4, which enables the AMD64 initiative with PCI Express support, and adds a twist in the form of SLI.
As mentioned, there are three versions of the nForce4 chip being introduced by NVIDIA: the nForce4 SLI, nForce4 Ultra, and nForce4. Each bears slight modifications that are intended to attract the attention of everyone from the most extreme gamers to mainstream buyers exploring lower-end Athlon 64 options.
Clearly, the most highly anticipated model is NVIDIA's nForce4 SLI, which features 20 lanes of configurable SLI connectivity. The above illustration demonstrates how the x16 slots on an SLI motherboard can be electrically set to run one x16 or two x8 links using a slot-type adapter. In the former configuration, the chipset recognizes a single card from either NVIDIA or one of its competitors. SLI mode is limited exclusively to compatible NVIDIA graphics cards, meaning they have to both be PCI Express (naturally), must be from the same add-in board manufacturer, and of the same model number (ie. GeForce 6800 Ultra).
Initial performance numbers with an SLI setup are promising, even if they were run by NVIDIA. Using an pre-production board from ASUS, and Athlon 64 4000+ and a pair of 6800 Ultra cards, NVIDIA claims that performance in Doom 3 at 1600x1200 with 4xAA jumps from 42.4 frames per second to 71.1, an increase of 68 percent. Though not nearly as relevant, the single 6800 Ultra scores 5211 in 3DMark05, while the SLI rig peaks at 9297, a 78 percent margin.
The chipset's other four PCI Express lanes are used for x1 peripherals. Interestingly, because the nForce4's switch only supports four connections, one of the lanes goes unused, even when the chipset is fully utilized.
NVIDIA's nForce4 Ultra also boasts 20 lanes of PCI Express connectivity, but it's hardwired to support one x16 slot and up to three x1 connections. Again, that fourth lane isn't used. The same goes for the vanilla nForce4, which supports 20 lanes as well.