AMD have now announced that they won't be supporting Radeon HD 4000 series and below in Windows 8. Most people will read this and be shocked, but the GPUs are old now and Windows 8 by the time it comes out, will be considered a 2013 release.
Windows should ship with some form of support for legacy Radeon cards, but AMD themselves won't be providing future driver updates for those GPUs. AMD have made this quite clear in their press release:
Also with regards to Windows 8 support for the AMD Radeon™ HD 2000, 3000, 4000 Series of products; the In-the-box AMD Graphics driver that ships with Windows 8 will include support for the AMD Radeon HD 2000, 3000, and 4000 Series, and it will support the WDDM 1.1 driver level features. The AMD Catalyst driver for Windows 8 will only include support for WDDM 1.2 support products (AMD Radeon HD 5000 and later).
Also, "legacy" in AMD's book of definitions includes the Radeon HD 2000, 3000 and 4000 series of GPUs. Beginning with May, those GPUs are only going to receive driver updates on a quarterly basis. This will allow AMD to focus on their newer, better, stronger cards.
We will continue to support the mentioned products in our Catalyst releases, but we're moving their updates to a quarterly basis, whereas our AMD Radeon HD 5000 and later products will continue to see monthly updates. The Quarterly Catalyst releases will focus on resolving application specific issues and critical updates. The reason for the shift in support policy is largely due to the fact that the AMD Radeon HD 2000, AMD Radeon HD 3000, and AMD Radeon HD 4000 Series have been optimized to their maximum potential from a performance and feature perspective.