suggest you read: http://linux.wordpress.com/2005/10/1...-installation/
you have to be careful (you have to reload the drivers if you [automatically] update ther kernel) I mention this in my post #150.
While the above is for Suse rather than Debian the basic premises apply.
Without knowing what processor, whether 32-bit or 64-bit, and the exact ati graphics card you have, it is difficult to advise you further, at this stage.
I also suggest you go to the LQ-Debian forum, where very similar threads exist, and are being more specifically answered today.