Update on my setup:
(on Linux, Ubuntu 10.10)
Although I thought I had 4 cards running at some point, one of the cards is bad, and freezes the system the instant I start *any* heavy GPU work (and I've isolated the specific card).
It's probably for the best, although the motherboard has 5 x16 length PCIe slots... without extenders you can only fit 4 cards directly on the motherboard, and then that's with no gaps between them. In my opinion, without some serious airflow around/through them they are going to overheat badly like that.
So I now have the same board, running with 3 cards, in the 1st, 3rd and 5th slots. With AMDOverdriveCtl I have MemClk set to 300, and CoreClk set to 960. That was just enough to reach 400MH on each card, 1200MH total. Temps all well below 70C on each card (far lower if I raise the fan speed).
What I love about AMDOverdriveCtl is the ability to manually define a Temp/RPM fan speed profile (curve). So you can easily trade of for noise vs. temps.
Temps of < 70C was easy with fans BELOW 50%. < 60C is reachable if you go to 70% or so. This is with an ambient room temp of 75+ F so you will likely do better in a cooler environment.
I have the cross fire connectors on but I don't beleive I have it enabled. I just fiddled until it all worked. Using "aticonfig --initial --adapter=all" was probably what did the trick for me. This creates an Xorg config file which activates all your cards, crossfire or not I beleive.
Now I just have to figure out how to either RMA or return that 4th card. *sigh*.
PS: I'm also running a 1200W supply which is now way overkill for this system, but at least its rock solid and silent.