Bunch of stuff in these comments...
- AMD release a new Linux driver at the same rate they do for Windows: monthly. Be it a bug-fix release or a new feature oriented package, it really depends on what they have in the pipe. The only variation there is is when an Ubuntu release approaches that bears a new version of the X window protocol, they may provide an alpha build of the driver to support it. Said alpha is then replaced with the stable version when it's out.
- both AMD and Nvidia spent a lot of resources sanitizing their driver development: 90% of the Linux driver uses the same code as the Windows driver - this includes the OpenGL part of the driver, as well as 2D/compositing desktop and video acceleration. However, while Nvidia dedicates its Linux drivers to professional applications, AMD does cater to big name games (case in point, several improvements that fixed gaming on Wine in 2010).
If you want a more in-depth view of Linux as an everyday environment, head over to http://www.phoronix.com.
@adamovera: for the HDD-to-USB copy tests, was the USB disk or key formatted in FAT32, NTFS, or ext2/3/4? The FAT32 driver in Linux is far from being optimized as well as ext, and NTFS on a USB key in Linux is a big no-no at the time (except when using the closed-source Tuxera driver, which is more the exception than the rule).