Since Nick is definitely right and Macs can be booted from any internal HD (MacPro) or any external one, you could install 10.6 (and update to 10.6.1 immediately) on an external Firewire 400/800 hard disk and see the new OS growing in time, along with software & driver updates. At the same time keep the current MacHD with 10.5.x for reliable daily use….time will tell when switching will be safe.
I enjoy both system in this way.
best
I
