i remember when i was running mandrake .. and i ran into this problem the drivers needed wernt out yet .. so i dont know if the x-series drivers are installed .. it has an option to auto config your ati card at login .. but if im reading your problem right your configing your card first before startx .? if so dont do that first start startx then connect to the net get the slackware version of the ati drivers and install them .. then config it .. that should work ...
hope that helps a little.
linux ati x800 driver h**ps://a248.e.akamai.net/f/674/9206/0/www2.ati.com/drivers/linux/ati-driver-installer-8.39.4-x86.x86_64.run


