Yeah, I was looking at the MSI 890FXA-GD70 and the CoolerMaster HAF-X. That combo should fit 4 cards. But Motherboard + Case is $365.
A cheap case + cheap motherboard could save $~250.
If I could get 2 5970s for under $600 each, then I'd come out ahead on the dual 5970 system with a cheaper motherboard and case.
Otherwise, assuming I could cool it, it still seems like the 4x 5870 system makes sense.
And yeah, I understand I could save more by building super cheap cases out of various other things, but I don't think I want to make that optimization.