The onboard USB controller causes a lockup on boot on the Asus M4A78 Plus. It's an AMD 770 chipset. The onboard NIC is also not supported. Disabling the usb controller completely and rebooting will allow you to get into the system with a ps/2 keyboard.
tags: motherboard amd hardware ati asus support
I recently purchased new video cards for my system. I wanted to get the new cards working in CURRENT with the latest version of X from mports. After several lockups and other odd behavior, I found that I had to disable the second graphics card (crossfireX in windows) in the X configuration along with removing the keyboard and mouse configuration. I was then able to start the X server with the radeonhd driver. Don't expect acceleration yet.