Recently I was search in Google how to enable direct rendering in Debian Lenny finally I found nice tutorial which guide to install fglrx then direct rendering will enable in your computer with any trouble. Link is below mentioned.
NOTE : not necessary to do the step 1.2 if you had correctly done step 1.1 (except step 1.2 other steps are required.)
Finally type the following command and verify direct rendering is enable in your computer.
#: glxinfo | grep direct
direct rendering: Yes