You will have to disable kernel mode setting first. It's a new feature present in the Linux kernel that is shipped with F12.
Modify your kernel boot parameters in the grub's configuration file with the "nomodeset" paramater.
After that, modify the "Driver" line in your xorg.conf file with "radeon" or "radeonhd" instead of "vesa".
Test which driver works best for you between radeon and radeonhd. I found radeonhd to provide better 3d support but exhibits more problems than the radeon driver.