SLI is supposed to work with WoW, but I wasn't able to get it stable with two ATI Radeon 4870s. I still have the two cards, but each is connected to a separate monitor. Usually I run my main on one monitor and the alts on another and that works fine. I've run all five on one monitor (and therefore one card) without any problems either, so having just one good video card should be plenty for 5-boxing, as long as the alt accounts are using the lowest video settings.