F18 New install, GUI doesn't load right

I just did a fresh install of F18 Desktop. Everything works fine on the live cd except that my dual monitors are reversed, but no big deal, right? Wrong! After rebooting the system boots but gets no further than displaying an incorrectly divided version of the desktop, and no graphical login prompt or anything. The incorrect division has like 3/4 of one screen belonging to the other, and the mouse will continuously slide through both if you move it to one side. I am able to open a terminal and log in, and xrandr displays "Can't open display". I used yum to install software updates from command line, to no effect. My graphics cards are dual HD 6850s (CrossFire), otherwise a typical system (fx8150, 8gb, dual sata hdds).

Any ideas? I'm stumped. I'll provide whatever information is needed on request. Cheers!