My 2 cents...
First, Logic 9 WILL run perfectly fine on 10.5.8... Leopard was my operating system for the past 3 years, until 2 months ago, when I finally upgraded my OS to Snow Leopard. Granted, I haven't updated 9 beyond 9.1.4, but 9 works on that OS, because that was the OS on all new macs when 9 was released 3 years ago or so.
You actually need Logic 7 to open any earlier version of Logic sessions (3, 4, 5 and 6), in version 9. At least that's the way I remember it. My advice would be to keep 7 around for a while, for this very reason... just to open Logic sessions earlier than version 7.
I haven't kept up with the MOTU hardware, but obviously you would want to make sure that there are up to date drivers for the 2408, before you update your OS