My graphic card is ati radeon hd 4570. i was getting an "no screens found" error message after "startx" and i couldnt be able to get in backtrack. (both 3 and 4). i search this forum and found some fixes. it was the error from graphic card driver, and i install my card driver. AND THIS IS HOW TO:
(FOR THE USB BT USERS. the cd users must remaster their cd.s)

1. Go to ati's site and and download the linuz driver for your ati g.card.
h p:// support. amd. com /us / gpudownload / Pages/ index.aspx
"delete spaces"

2. you will able to download like this : "ati-driver-installer-8-01-x86.x86_64.RUN" . after download , rename it as "ati.run" and copy this file to your BT4/ROOTCOPY folder.

3. boot your BT and write "root-toor" (admin you know). after that, DO NOT "startx". write "cd ..". so you will be in the "rootcopy" folder.

4. then write:
bt ~ #chmod +x ati.run

bt ~ #./ati.run

5. then "enter-enter-enter.." until the installing starts.

6. now write startx and enjoy it. )