I finally managed to solve my problems with the Windows Radeon Drivers on my MacBook Pro. When in Boot Camp, I was never able to set the resolution of my external monitor right, even though it worked in OS X.
I first tried to install the Radeon Omage Drivers from here, which helped a little, but still didn’t really work right: I was able to set some resolutions, but not all, and especially not 1680×1050, my external monitor’s native resolution.
The solution for that is simple: In Display Properties | Settings | Advanced | Displays, un-check the checkbox marked “Preserve Wide Aspect Ratio on attached displays.” Then it just works.
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