Right now you can use two different drivers for your Radeon card.
One, is an open source, free driver that comes with the X window server.
Two, is a binary only, driver that ATi provides.
With one, you are using GPL code but don't get as much bang as...
Two, where you get more bang but are not fallowing the Free Software ideals.
Right now you have one. So going to ATi's site is of no use to you.
I'll give you a hint, you are looking for something called xinerama.
Mandrake most likely set this up for you, however you need to change a setting to make it work.