You might want to verify that motherboard is crossfire capable before recommending dual AMD video cards. This IS a 6 year old computer we are talking about....
Adam, if you want to reduce power consumption, I would recommend ordering an HD7850 or HD7870 on Monday. That way, when you upgrade in the next year or so as parts in that old machine start to die, you can transfer the HD78XX to your new machine and purchase a 2nd to put in crossfire.
Well he currently is using 2 Nvidia 7800's so chances are it supports it !! (EDIT : though you're right it might only support SLI and not Xfire !)
Twin Nvidia 7800GTX graphics (one is cooked)