For Motherboards have a look here:-https://bitcointalk.org/index.php?topic=37880.0

For info on which Motherboards need pins shorted to run PCIe Extenders, look here:-https://bitcointalk.org/index.php?topic=36061.20

Be aware that Gigabyte M/Bs can be a PITA to boot from a USB Stick.

Dual GPU Cards seem to have a reduced life expectancy compared to Single GPU Cards --- heat issues

 5870 or 5850 Reference cards are my preference.