Just finished this one yesterday. My first rig was in a nice case and all but this one I was trying to save as much money as possible. The only things I bought were 2 x 6950's, 2 PCIe to PCI32 adapter cards and an ANTEC 750 WATT PSU. The motherboards are some old crap ones I had lying around. One is out of a DELL computer as you might recognize and the top one is a shitty PC Chips mobo (hate this motherboard, the only thing its good for is mining).

The top PSU is an old ULTRA 700 WATT and is powering the hard drives, fans and top motherboard. The bottom PSU is the new ANTEC which is powering the two 6950's and bottom motherboard. At first I tried running 1 card off the old Ultra but that didn't smell to good lol.... I grounded the two together to try and illuminate any floating point ground there might be. So far its been mining straight for about a day with no problems. On the right side is a breadboard circuit I quickly threw together to start both at the same time. I first was just pushing both power buttons at once but that gave odd results sometimes if you were off .

Currently the DELL motherboard is beating the PC Chips board at 343 Mhash/s at 900 MHz core. The PC Chips board is at 336 Mhash/s with 900 MHz core. Both can go a bit faster with -f 1 but I haven't test that to much yet. Both running Ubuntu 11.04, SDK 2.4 with latest drivers. The temps are nice, top card is ~62C and bottom card is ~58C with fans at 65%.

All in all I'm pretty damn proud of the monster (or mess) I've created.