you need to compile the ATi driver for Debian 3.1 Sarge and then use fglrxconfig to setup the graphics card.
Here is the link to the site, follow the instructions exactly, I am a Linux n00b but I was able to compile a driver to work with my Debian install so I had big desktop and OpenGL 3D graphics.
I downloaded the source files in step 3a, the binaries failed on my install but might work for you (in that case you can skip most of the steps).
In step four you need to compile the kernel module (required for 3D graphics), I used one of the prebuilt kernel modules available at this link http://xoomer.virgilio.it/flavio.stanchina/debian-fglrx-modules/index.html
(you should be able to workout which is the right one for your kernel); then skip to step 5 and you should be ready to go.
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