Hi Roland,
regarding the XBMC version - try to use Deb-multimedia repository [1]. There are more recent versions available.
Ad 1) more things which needs to be checked
- kernel support
- Xorg open source radeon or closed sourced fglrx ati drivers support
Try to read these [2] [3] [4]
Ad 2) first make sure your direct rendering is working (running the command "glxinfo")
then you can just install xbmc and some other packages, try to use aptitude [5] which have some console-based menu for package management.
There are more xbmc-related packages - search for them and read about them for more information. Decide what's needed by yourself.