If you are having performance troubles with Radeon cards and FarCry 2 or S.T.A.L.K.E.R.: Clear Sky, AMD has a hot fix out for you.  It is a beta driver and is unsupported by AMD, so you may find it causes other problems for you.  Be warned before you try.
This hotfix
  • Improves performance for the DirectX 9 version of Far Cry 2 on CrossFire compatible systems
  • Improves performance for the DirectX 10 version of Far Cry 2 on both single and CrossFire compatible systems
  • Improves performance for the DirectX 10 and DirectX 10.1 version of Stalker Clear Sky on both single and CrossFireX configurations at higher resolutions

The hotfix download package includes both the Display Driver and Catalyst Control Center. The hotfix can be downloaded by clicking on the links below.

Far Cry 2 and Stalker Clear Sky XP HotFix (includes XP 32-bit and 64-bit)

Far Cry 2 and Stalker Clear Sky Vista HotFix (includes Vista 32-bit and 64-bit)

Note: This driver is provided as is and is not supported by AMD. It has not completed full AMD testing, and is only recommended for users experiencing the particular issue described above.