Hello, I tried to update an older laptop (4 yr old HP) at the weekend with Sabayon 8 KDE & got the black screen. Tried text install, graphic install etc, different boot options ... I did Alt - F1 to the terminal & ran installation (think the command was "installer" although I cannot be sure. Installation ran, it went as far as copying files & just stopped. Tried v7 KDE & it was a similar story. Tried openSUSE 12.1 & it installed 1st time in graphic mode. These were all 32bit DVDs.
I run 64bit v7 Sabayon KDE here updated to v8 on a new HP laptop & installation was a breeze. I cannot help you but you are not alone.
Shame I couldn't get Sabayon on my Mums laptop but openSUSE is a decent OS