Hi Lefty,
I take it you mean the AX1200 or AX1200i, both of which are compatible with pretty much every motherboard ever made, including the AS Rock Extreme 9.
However my PSU of choice for this Wattage would be the Antec HCP-1200. It pretty much always beats the corsair units in tests, and has been seen running very stable at over 2000W. Its cheaper too
http://store.antec.com/Product/HighCurrentPro/hcp-1200/0-761345-23920-2.aspx
Its not fully modular, but the attached cables are chosen very sensibly, I'm using all but the extra 8-pin power connector which is designed for dual CPU boards.
Just out of interest, what is the rest of you build?