I would get it stable without overclocking and tweak it from there. OC will make it even hotter and most go without a case especially with 4 GPU.

I think that may be a good call on the 3x boxes if you don't forsee problems mixing the 5870s and 6990s?  I don't want extender cables if I don't need them and the 6990s arrive a couple of weeks after the 5870s so I could have two boxes running 2x 5870 and then plug a 6990 into each when they arrive.

Hi Everyone, I've been reading the posts and thanks for the great info, I'm ready to get started on my build over the next few days as the parts start to arrive.  My root question is whether to setup 1 rig with 4x 5870s and another rig with 2x 6990s or to create 2 boxes with 3x GPUs each...  Here are the main parts on order:

4x VisionTek ATI Radeon HD 5870 Eyefinity 6 2 GB GDDR5 PCI Express Graphics Card 900322
2x AMD Radeon HD 6990 (These are due to arrive in 2 weeks)
2x MSI AM3 USB 3.0 + SATA 6Gb/s AMD 890FX Chipset Motherboard 890FXA-GD70
2x Corsair Professional Series Gold High Performance 1200-Watt Power Supply CMPSU-1200AX
2x Corsair Vengeance Blu 8 GB PC3-12800 1600mHz DDR3 240-Pin SDRAM Dual Channel Memory Kit
2x AMD Sempron 140, Socket AM3, 2.7 GHz, 1 MB Cache, 45 Watts (I may buy fancier procs in case I ever want to repurpose these)

I'll use Ubantu on a USB stick instead of a hard drive and I'm hoping for advice for the specific cables I'll need as I think that is dependent on the rig setup decision.  I think my preference is to pack one rig with 4x 5870s and then run the second rig with 2x 6990s so that I have room to expand on that board.  Any thoughts?