Comment 38 for bug 341681

Alex, I vaguely remembered that we had the same issue with radeonhd with an Asus Laptop, that only worked with only 2GB installed.

Now, found on our wiki http://www.x.org/wiki/radeonhd :

Newer ASUS laptops with more than 2GB of memory. There are several reports about getting RadeonHD (or any graphics driver) to work on Linux on ASUS laptops with M72, M82 and possibly other chipsets when more than 2GB of RAM is installed. This is very likely not a graphics driver issue. For now the only work around is to reduce the amount of RAM installed to 2GB or less.

AFAIK we still weren't able to find a reasonable solution for that.

Just for information