Download the latest driver from
http://ati.amd.com/support/driver.html
You need to choose Linux_x86 -> Mobility Radeon -> Mobility Radeon X1400
The file will be called ati-driver-installer-8-9-x86.x86_64.run
In a terminal window run as root
sh ati-driver-installer-8-9-x86.x86_64.run --buildpkg Slackware/All
Two files will be produced:
fglrx-module-8.532-x86-1_kernel_2.6.24.5_smp.tgz
fglrx-x710-8.532-x86-1.tgz
They are Slackware packages. Install them with the installpkg command and reboot.