After successfully trying the AMD Catalyst driver (a.k.a. fglrx), I decided to go back to the open-source drivers for several reasons.

• A certain memory allocation failure from X.org scared me a bit.
• The proprietary drivers don't play well with SuperTuxKart (e.g. Debian bug #586797) causing texture corruption both with 0.6.2a and SVN trunk.
• Tearing. Lots of tearing with 2D. Forcing use of VSync in the configuration causes increased CPU usage for some reason.
• XVideo acceleration consumes some more CPU time than the free drivers' code for some reason (uh...).
• I discovered that the screen or GPU buzz when running on batteries after all.

There isn't anything in fglrx that I really needed to have, so I'm sticking to the free (open-source) drivers again, most likely for the rest of the laptop's lifetime.