Nvidia has updated its multi-GPU SLI technology to allow for three-way setups, in addition to the usual two or four-way ones. According to the company, combining an nForce 680i SLI mainboard with three supporting cards should give you a 2.8x performance increase over a single GPU system.
Unfortunately, that will come at a pretty hefty cost as you'll either need three GeForce 8800 GTX or GeForce 8800 Ultra graphics cards - that's about $1500 to 2000 in just GPU costs, not to mention power, noise, and temperature concerns. Even though Nvidia has just released the new GeForce 8800 GT and 512MB GeForce 8800 GTS, neither of these cards will work in a triple SLI configuration.
You can get your own pre-built three-way SLI machine from Alienware with the Area-51 ALX SLI, which comes equipped with a trio of GeForce 8800 GTX or Ultra cards and an overclocked quad-core CPU inside, for the obscene price of $4,999.