Remember SLI for the Voodoo 2? Where you should sort of chain two cards together to achieve higher resolutions? Well, Nvidia is reviving the concept - after a fashion - with another SLI - Scalable Link Interface.
The idea is simple: 3D graphics is a highly parallelizable application. Current generation GPUs all consist of variable numbers of parallel computational units anyway. Why not link them together in a way to take advantage of more cards? At the purely technical "gee whiz" level, this is a pretty cool idea. In fact, Nvidia has even built support for this into their NV40 GPU die. A small percentage of the die is allocated to a high speed I/O port that can be used to communicate between other NV40 GPUs.
Then of course you have the inevitable drawbacks starting with double the price (for two cards obviously) but more importantly, the need for a platform to support two PCIe graphics cards... overall however NVIDIA has made a step in the right direction, either if you want to look at this as a way to build the latest gaming machine or a great alternative for future GPU upgrades. More here.