Update to this:
I ran CrossFire on my (2) 6990's for a couple days. It worked well - but I had to run my fans up quite a bit to ~70% to keep the cards around 80*C. Even with my Antec 1200 case, these cards still get crazy hot! I decided that I wanted to trasnfer the hardware out and put my GTX285 back into my main computer, and build a dedicated mining rig instead.
I ended up going to MicroCenter on Sunday and picked up the following. It's a nice cheap build, but very powerful and has plenty of room for expansion. I went with quad-core on the CPU in case I decide to use this machine for a web/dev server.
MSI 890FXA-GD70 Motherboard - $169.99 after rebate
CORSAIR XMS 4GB (2 x 2GB) RAM - $44.99 after rebate
AMD PHENOM II X4 840 CPU (Quad-core 3.2GHz) $97.99
Total cost: $312.97
The motherboard has 5 PCI-E slots. I'm only using 2 of them right now, and could probably fit a 3rd 6990 if I really wanted to. If I had a cooler running card, I could probably stack 4 5830/5850's in there, without issue. I ended up getting 2x2GB for RAM so I can use the other DIMM in another rig if I decide to build another one. I put all of these components along with my Corsair 1200 PSU into an old Chieftec case that I had lying around. I also put in an old WD 250GB HDD, and a 3TB Hitachi HDD, since I can now use this as a network backup machine as well. I installed Ubuntu 11.04 x64 Desktop, and I'm running the miners via poclbm right now.
I'm getting around ~324 Mhash/s on each GPU core. In Windows 7 x64, I was able to get ~350 Mhash/s using GUIMiner, so perhaps there's some tweaking that needs to be done.
My new issue is heat. This old Chieftec case does have 5 fans, but they're all small 80mm ones. I have a couple extra 120mm 72cfm fans lying around, so I'm going to try and position those effectively. The case is running at home now with GPU cores @ 77c - 79c - 80c- 74.5c, fans set to 85%. It should be noted that my side panel door is also open, but not completely off. It's resting against the case and the opening is towards the rear.
I'm also thinking about going caseless, since it looks like it will help quite a bit in terms of heat dissipation. The MyOpenPC test bench also looks VERY nice, found here: http://www.amazon.com/MYOPENPC-Transparent-Acrylic-Stackable-Capability/dp/B004TZAU1Q/ref=sr_1_11?ie=UTF8&qid=1306839609&sr=8-11
Perhaps I could get a couple of those and stack them. (if I had multiple builds) Another option is to buy some 4U rackmount cases. I find the mining process to be very interesting, and would love to build 2-3 more machines using the same components that I picked up this weekend for cheap. I could then rackmount all the machines on a server rack at home, or even get some colospace in a datacenter. I'd have to make it worthwhile to go that far though.