Since I installed Gentoo Linux a few weeks ago, I have made it a habit to check the ATI website almost every day to see if a new driver has been released that supports my AMD64 processor as well as the XOrg (instead of XFree) server. The default driver works fine in 2D, but it does not support 3D acceleration.
Today, my search was finally rewarded. Here is the updated ATI driver. Of course, there are a few known issues, so I should probably surf around and read the initial success / failure reports before attempting to install the driver myself. Also, the new driver has not made it into the Portage tree yet, so I guess I’ll have to install it manually.