Besides dozens of small fixes, these are the three biggest changes:
OpenGL CrossFire Support Under Windows Vista
Catalyst 7.3 introduces OpenGL CrossFire support for the entire ATI Radeon X1000 product series under Microsoft Vista. Users will see significant performance gains in a number of applications when running in CrossFire mode under Windows Vista.
CrossFire Enhancements for Windows Vista
Catalyst 7.3 includes CrossFire enhancements for Windows Vista that enable the Alternate Frame Rendering (AFR) CrossFire rendering mode by default, enabling performance benefits for all AFR compatible applications immediately (including newly released applications that have not been officially profiled for CrossFire). Future Catalyst™ releases will add further CrossFire performance gains once new applications have been profiled specifically for CrossFire.
Dark Messiah Might and Magic performance improves 12.5% or greater on all Radeon X1000 series configurations when running in CrossFire mode under both the Windows XP and Windows Vista operating systems. Larger gains of 60% or more can be seen on high end products.
Download over at ATI's website.