[Slim Shady]

Jesus would check the compatibility documents (link at the top of the page) and not upgrade to something that was unsupported by digi unless his rig is not currently in a "must-work" situation, and he has time to experiment and possibly re-install if things go terribly wrong. If you can't afford any downtime, then you shouldn't upgrade anything EVER unless you're ready to deal with the worst case scenario. It rarely happens, but you've got to be prepared for it if it does.

On a personal note, I'm running 10.3.6 and PTLE 6.4 runs fine with my Mbox, but in general 10.3.6 is a crappier upgrade than 10.3.5 was - Apple's really had a hard time with the panther upgrades, here's hoping Tiger will be better.