In my experience the biggest draw on boxing is cpu and memory. A mid range video card is going to be just fine without running SLI. The card you are running is probably well beyond overkill for 5 boxing one pc. As already stated the best way to go for 5 boxing is to get a quad core. I just upgraded from a dual core to a quad core yesterday and I now have zero lag or stuttering in shattrath.

Edit: I'm only running 4 on one machine. I like running my 5th guy on a seperate computer although with the quad core upgrade I have no real need to do this.