Not coincidentally, a new driver has been released today. Catalyst 10.9 offers performance improvements for S.T.A.L.K.E.R.: Call of Pripyat and Chronicles of Riddick: Assault on Dark Athena for certain 4000 and 5000-series cards. There are also new or updated Catalyst profiles for Aliens vs. Predator, Battlefield: Bad Company 2, F1 2010, and Kane & Lynch 2.
There are nearly a dozen highlighted bug fixes in the release notes (PDF), spanning Windows XP through Windows 7, and Catalyst 10.9 brings official support for hardware acceleration in the Internet Explorer 9 beta (which is now available). Look under the "Steam" menu in the top left of the Steam client for an option to update your video drivers, or just download them below:
Desktop: Windows XP 32-bit | Windows XP 64-bit | Windows Vista/7 32-bit | Windows Vista/7 64-bit