Tech-report.com's Scott Wasson has written a nice article on his experiences with Nvidia SLI. Nvidia SLI is a means by which one can make two Nvidia cards work in tandem to accelerate graphics and games on a single monitor using two graphics cards.
"NVIDIA's SLI tech is about as cool as it gets, and I wanted to see what happens when you pair up a couple of GeForce 6800 Ultra cards for a PC graphics solution nearly twice as powerful as anything else available."
Performance is revealed to be impressive to say the least; however the author makes mention of system stability issues that still need to be resolved. Mostly, however, these seem to be heat related and can be solved without too much hassle. Go here to read the article.