Or that, though a third GPU gives absolutely abysmal returns... Say going from 1 to 2 gives you some 85% gain, going from 2 to 3 only yields another 25% or so. But since he already has a 5870 it's an option; no money lost there.
Again though, that 1GB VRAM can't cope with 5760x1080, so I think it would only hold it back.
*and that PSU will fall short