You do not need an SLI board, just one with two PCI-E slots. Even a 16x/4x configuration will do. If you don't care about the gaming performance though you should know that you can use a single Radeon card with Eyefinity to drive three monitors. This can be done on a lowly 5670 provided it has a display port. Sapphire also makes a line of "Flex" cards which do not need an active display port adapter to run eyefinity.