I don't know much about SLI.. is it limited to two cards? An even number? A base of two?
SLI is two cards. There are similar technologies that have (somewhat) recently been released by nvidia to link together four cards. I'm not sure if it's under the name SLI, but I sort of doubt it has the exact same name.
Unfortunately, the Quad SLI implementation as it has been done so far does not actually link together four physical cards - rather, it links together four physical GPUs, two of which are present in the NVidia GeForce 7950 chipset.
The primary drawback to this is, of course, bandwidth. Using four GPUs across two cards, or two GPUs in one card, requires double the bandwidth to push data down the pipe.
I'm considering buying another 8800 GTX when I've got some extra cash laying around and there are drivers for it. I hate NVidia right now because the only DX10-compatible card on the market, the 8800, does NOT HAVE DRIVERS FOR VISTA. UGH. Then I'll do dual-SLI.