Everything was good with my dual 5850 until I added 4 more. Ubuntu Natty simply stuck at a blank screen (without internet connection) after the logo "Ubuntu" is loaded. I tried the following (through recovery mode):

1. Reinstall the driver: tried auto install from Ubuntu, and from the package downloaded from ATI website. Also tried manual install.
2. aticonfig --initial -f --adapter=all
3. aticonfig --initial -f --adapter=0,1,2,3,4,5
4. In /etc/X11/xorg.conf ServerLayout section, change "RightOf ...screen...1", "RightOf ...screen...2..." .... to "RightOf ...screen...0..." and other layouts (also tried set resolution to 800x600)
5. In /etc/X11/xorg.conf ServerLayout section, change "RightOf..." to 0 0

None of them work, I couldn't enter login screen unless I run "aticonfig --initial -f" which only enables the first device. It doesn't solve the problem.
Of course, all cards can be detected by aticonfig (aticonfig --list-adapters), and atiflash in MS-DOS.

GPU-Z can detect all 6 GPUs on Windows.
Motherboard: GA890FXA-UD7

I think a small change in xorg.conf should be able to fix this - for the moment, I am trying to increase the number of adapters one by one. But it seems Ubuntu couldn't even start with only 2 adapters (aticonfig --initial -f --adapters=0,1)