I got my new 2.66 Quad Core some weeks ago with 10.6. Updated to 10.6.1 as it became available. I've noticed the temperature climbing when using Safari, and - at least twice a day - the system hangs when shutting down or put to sleep, forcing me to push the power button to restart it. Sometimes I can see that'll happen when a beach-ball hangs in the menu bar area. Seem's to me 10.6.1 is not stable. I restored 10.5.8 from a SuperDuper backup I had from my previous iMac, to a 2nd internal drive, and it seems to boot and work OK... temperatures seem lower under Safari+Flash.... BUT: ... ¿Are there any negative risks or side effects of running 10.5.8 from an iMac restored backup in my new MacPro that I should be aware of? for instance - any hardware drivers missing specific for the MacPro? I read somewhere any new mac cannot run older OS than the one installed - specially when it's a major version (i.e. Leopard to Tiger)... perhaps this doesn't apply when restoring from a backup? Any thoughts or advice? Thanks!