Think in terms of scaling and supporting hardware price per hash. You want a standard box with no screwy extenders or anything like that. I can tell you from experience that if you put 3 or more 5970s on a single motherboard and overvolt them you're going to burn some pins off your ATX connector, but that's only something you'd probably do with water cooling. I'm not sure about motherboards with more slots, but in my experience the problem is with the ATX connector on the PSU side. I've burnt the plastic off 3 12v pins already.
Basically if you go desktop the most you can fit in a standard configuration is 4 dual-gpu cards, and in a rackmount server the limit is 3. Again, you may have PSU issues.
And yeah, the rest of the parts don't matter. In Linux if you use the right OpenCL stack you should have little to no CPU usage from the mining anyhow.