Mac OS X 10.5.5
Limited testing with Pro Tools HD, LE and M-Powered 7.4.2 with Mac OS X 10.5.5 shows reliability and performance is at or near parity with the same systems on Mac OS X 10.5.3 and 10.5.4. Pro Tools 8.0 will be fully qualified with Mac OS X 10.5.5.
If you decide to upgrade your operating system to Mac OS X 10.5.5, the "Combo Update" installer linked below is the recommended method of updating your Mac OS from an earlier version of 10.5:
* Mac OS X 10.5.5 Combo Update
Updating your Mac OS using the built-in Software Update or smaller incremental "Delta Update" downloads may not install all required components, and is not recommended. If you have a new Mac that shipped with Mac OS X 10.5.5 already installed, you do not need to "downgrade" your operating system to a lower version.