I just tried to use the pci-e 1x for another 5830 in my 2x5830 rig using a 1x to 16x cable (which would make it into a 3x5830 rig).
I had partial luck...
The rig runs LinuxCoin. What happened was "aticonfig --lsa" listed all three devices, but "AMDOverdriveCtrl -h" only listed the ones plugged into 16x slots, even though it had a couple more entries in its table than usual.
When I tried to run phoenix miner on the DEVICE 2 that "aticonfig --lsa" listed, it tried to use the CPU...
So, yeah the pci-e 1x to 16x cables DO work, but if anyone could help me with my particular LinuxCoin issue, that'd be great!
I can offer .5 BTC for a complete solution.
If I was in that position I would
1. rm /etc/X11/xorg.conf (path might not be exact)
2. reboot with dummy plugs in the cards.
I know you dont normally need the plugs in the cards, but I do remember a situation that cleared up when I used them. worth a try