In order to install the nvidia graphics drivers on BT5 you need to enter the following into a terminal prompt while X is not running.
First you need to prepare the kernel sources if you have not done so already by:
Please remember to copy and paste these commands into your terminal prompt as formatting can make long lines wrap over into two or more lines !
apt-get install linux-source
cp -rf include/generated/* include/linux/
Then for 32bit BT5: following the prompts
chmod +x NVIDIA-Linux-x86-270.41.06.run
Or for 64bit BT5: following the prompts
chmod +x NVIDIA-Linux-x86_64-270.41.06.run
And that's it, you should now be able to restart, startx and enjoy the full glory of BackTrack at the full resolution offered by your graphics card and monitor.