I just built a computer with two sapphire HD7950 graphics cards. I've tried installing both lubuntu and xubuntu (for scrypt mining) and in both cases the same thing happens. After the initial splash screen that just shows the logo and a loading thing, it goes to a black screen with a cursor.
If I CTRL+ALT F4 for example, it brings me to a xubuntu@xubuntu terminal window. I wouldn't mind that much only having a terminal, but I'd like to know how to get the GUI back.
EDIT: I have also tried with lubuntu the "nomodeset", which just displays a whole bunch of errors, most of them say SQUASHFS error: Unable to read data cache entry