Wow, thanks people, this is some seriously helpful linfo/advice.
I should've mentioned that due to my peskily TIGHT budget this is a used Mac Pro indeed - a 2008 3,1 in fact.
I am definitely SOLD on sticking with 10.6.8 for now, thank you, and based on PanCenter's post last night I clean installed 10.6.3 from the supplied DVD, along with Rosetta, on a new drive on the new (to me) Mac Pro, then updated to 10.6.8. I'll also make sure to go with Logic 9.1.5 (thanks for update link, Art Of Sound!!)
I'll have to vow to do the disk cloning thing after getting everything running nice - I haven't used Time Machine before, but it looks like it's safe to assume ya still gotta go with Super Duper or CC if you want a complete system restore with all authorizations...which of course is a critical requrement.
So it appears as if there are 2 migration schools of thought:
1) Try the easy breezy MA on for size, with the intent of reverting to a clean OS re-install if issues arise.
2) Just hassle the dang hand copying clean install of everything right from the get go in order to be 100% confident, and well...clean. 😝
Next up - I'll be reading that basic guide for migrating - thanks for the link MichaelLAX. 😎
Will probably stiil plan to keep the PPC 10.5.8 machine around. Lordy those really can be soild indeed PanCenter. I feel like I should award it a gold watch and a pension. 😁