Running into this with the edgy install CDs, had to get the alternate install CD and then use a VGA mode before I could install onto an Dell Optiplex GX620. PCI-ids for the X600 PCI-express inside are the same as those in comment 15, switching from the 'ati' driver to the 'radeon' driver post-install gets me a useable desktop (although I've not tried 3D yet).
-- X fails to start in dapper, edgy with ati X600/X700/X800 (control not delegated to "radeon" driver, "ati" complains about Mach64). https://launchpad.net/bugs/28925 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
