These are the files i patched to have my Radeon X1800XT with 512MB VRam work with QE/CI working. In Lion Quartz 2D is enabled but in Mountain Lion only QE/CI is enabled.

Just use kext helper or next utility to install and reboot.

They are for X1800XT 512MB with user Device ID 0x7100. See Screenshotssource