you could install read-edid mdetect hwinfo and xdebconfigurator to setup x for you but you will still need
http://wiki.serios.net/wiki/Main_Page for ati driver guide. There may be some sarge amd64 specific tips you need. Check what kernel version,what kernel config settings,what XF86 version or xorg version is needed for your video card.You even have Fedora XF86Config-4 file for help.
Luckily there are many threads in these forums for that card.
Just go and search. Good luck.