This is only tested on Radeon HD 6470m
the fglrx drivers are know NOT to work in backtrack most people force the i915 drivers instead.
this is from a fresh installed backtrack 5r3

1. apt-get install python-software-properties
2. sudo add-apt-repository ppa:glasen/intel-driver
3. apt-get update && apt-get upgrade
4. download the radeon drivers from atis website
5. install them
6. apt-get install xorg-video-radeon
7. edit your /etc/default/grub find the line GRUB_CMDLINE_LINUX_DEFAULT and change it to
GRUB_CMDLINE_LINUX_DEFAULT="text splash video=radeon:modeset=1"
8. add "blacklist nouveau" to "/etc/modprobe.d/blacklist.conf"
9. update-initramfs -u
10. update-grub
11. reboot

i don't really know if ALL this steps are necessary but for the first time i have the fglrx drivers working, with dualscreen and full resolution