After installing Catalyst 5.9, I have the white line too in non-native resolutions. It's really annoying, because I have so many games that will not run in 1280x800.
PS. I don't have an Acer notebook, i have aa ASUS A6Va notebook.
I will try some older drivers.
Edit: Tried older drivers, and even with Catalys 5.2 I still have the white line, even in Windows in non-native resolutions. But when i uninstall the drivers, and i set the resolution to 1024x768 and other non-native resolutions, I don't see the line. It must be a driver problem i geuss.
Edit2: Installing Catalyst 5.8 with Catalyst Control Center, solved the problem here. Catalyst 5.8 with the Control Panel works fine here. For all with the white line problem, try installing Catalyst 5.8!