I also had the same issue with the Radeon HD7750, so here is what I did:
So when you boot up, you should see the GRUB menu with Start Sabayon 14.08 KDE selected (depending on your image). You have two options here. The easy way would be to select Advanced Options and then disable kernel mode settings and then hit enter.
Press e to edit
go to the line with linux /boot/sabayon aufs ...
change the line so it reads:
linux /boot/sabayon aufs cdroot splash quiet xdriver=versa vconsole.keymap....
So, as you will notice, the only thing we have added is the xdriver=versa to the above line
press F10 to boot
Either option above will get you booted back out to the terminal as you were before, but this time will be different.
#systemctl restart kdm
Alright, the above info is assuming that in your bios under boot, CSM you have UEFI Only. If you are using Legacy OpROM only then when you boot to the USB you will actually see the sabayon menu. At this point you can press F5 for additional parameters, select nomodeset using the space bar, esc back to the menu, and Start Sabayon 14.08. This will boot you out to the terminal and then you run the systemctl restart kdm