The size of the update may vary if you pick it up via Software Update -- we saw 315 MB on a MacBook Pro running 10.6.3. The combo update for all 10.6 systems weighs in at a bandwidth-hogging 887.4 MB.
Here's Apple's write-up on the matter:
The 10.6.4 Update is recommended for all users running Mac OS X Snow Leopard. It includes Safari 5 and general operating system fixes that enhance the stability, compatibility, and security of your Mac, including fixes that:
- resolve an issue that causes the keyboard or trackpad to become unresponsive
- resolve an issue that may prevent some Adobe Creative Suite 3 applications from opening
- address issues copying, renaming, or deleting files on SMB file servers
- improve reliability of VPN connections
- resolve a playback issue in DVD Player when using Good Quality deinterlacing
- resolve an issue editing photos with iPhoto or Aperture in full screen view
- improve compatibility with some braille displays