My first question is about the antialiasing feature of
the ATI Radeon 9000 (called smoothvision):
How can I use/ activate this feature?
The environment variables for NVidia cards are well
documented but I can't find anything for ATI !

Secondly, I would like to use the dual monitor feature
of this graphics card. Activating it using the ATI Control
panel under KDE doesn't have any effect. In fact, when I start the X server I only have output on one monitor, otherwise
I get output on two monitors.

I use the Linux drivers provided on the ATI webpage.

Any help is appreciated

I too am interested in knowing how to enable FSAA on my Radeon 9700 pro. Is there a setting in the x config file for that, or is it in the wine settings somewhere? I currently only have wolf et installed, and would like to enable fsaa as well as antisotropic filtering for my games.

aticonfig should be able to enable fsaa for your ATI card... of course this is assuming you have that package/module/whatever-they-call-it-in-*ix that gives you the aticonfig command- Back up your current xorg.conf file- make sure ATIConfig is set to modify the correct file and just pass it the flags:
sudo aticonfig --initial --input=/path/to/xorg.conf

If that works, use aticonfig again but pass it the following flags instead.
--fsaa=on
--fsaa-samples=x (x is 0,4,6) you get the idea.

Im somewhat of a neophyte with Linux and how to restart the X-server etc. etc. so my half assed way of checking it is an init 1 followed by an init 5... your mileage may vary.

Now then- it is important to note I have gotten fsaa to work- and seen it work on such things as GLX_GEARS, however- whether it will work with XGL/Beryl/Etc. is questionable... I seem to be getting hangs/X-server crashes when I try to load it up with FSAA enabled.

Hope this helps- and good luck.