GPT refers to the GUID Partition Table (Globally Unique Identifier...you know things are getting nuts when they start making acronyms out of the acronyms...), which is the partitioning scheme used by the newer Intel machines. Pre-Intel Macs use APM (Apple Partition Map).
There seems to be a great deal of argument about whether or not it is possible to boot an Intel machine from an external drive formatted with APM. I haven't gotten to the bottom of the arguments yet, though it would appear that it is possible, providing you're using a Firewire drive. What you need to do beyond that I haven't yet sorted out. At any rate, it isn't the DRIVE but the partitioning that may cause problems - and that can be solved using Disk Utility. No need to buy another drive, though if you have older Macs as well, you'll need to figure out what to do if you want to use the same drive to back up more than one machine. If I figure this out (because I am now in this same boat) I'll let you all know. If anyone has a good, uncomplicated source of RELIABLE instructions, please post them. I've found too much arguing so far...
