SLI doesn't really require anything so to speak, just an SLI ready motherboard, SLI ready graphic cards, and a PSU which can deliver enough power to those cards. Also a 64-bit system is recommended, mainly due to the memory address issues. You can SLI different brands (different memory sizes are a bit more difficult I believe and you'll need to do some tweaking) however you must SLI the same GPU. For example, an 8800 GTX with an 8800 GTX, not with a GTS or Ultra. An exception is the 9800 GTX+ with the GTS 250, as the GTS 250 is merely a rebadged 9800 GTX+.