Definitely check @tinhed answer first. But after that, try the bleeding edge Xorg from the xorg crack pusher team. Performance with my Radeon x300 shot way up. The compiz window switcher now produces a 3d holograms as the screen flips around workspaces, and with a python module it can enlarge my screenspace without changing the resolution or dpi.
Bear in mind the stern warnings:
Packages for those who think
development versions, experimental and
unstable are for old ladies. We want
our crack straight from upstream git!
Well, straight, we want it built and
packaged so we don't need to know what
we're doing, except that we will break
our X and put our computers on fire.
Also, Xorg windows start shaking if I haven't run
apt-get upgrade recently.
May need to add yourself to the video group.