Had another thought after reading Bob's post.

Which software are you planning on using to MB? I'm planning on ISBoxer and one of the reasons I bought a second video card wasn't to use SLI (WoW doesn't support it), but Innerspace allows you to allocate video to different cards. So I plan to have my main toon use one card and my four slaves use the other, in a non-SLI configuration.

I'm not sure if IS supports three cards, so you may want to ask Lax about this.