I had the same problem on one of my boxes on a clean install. My other box installs flawlessly. On the first install it was fine, but I had display problems in KDE, so I uninstalled and upgraded my card to a ATI Rage Xpert 128, which was on the compatibility list and got to the part about setting up the X server and that's when I encountered the same message. The only thing I can assume is the driver isn't installed for that card or something needs to be configured by hand. Everything else installs OK.