Help with getting an ext hard drive for 2 comp's.

You could clone your G5's hard drive with SuperDuper or Carbon Copy Cloner, but that would be best down with an extra partition on the external drive, as cloning with Super Duper at least erases all data on the target drive.

You can find duplicates with a software called Find Duplicate Files (it's a demo, but you can use it for 7 days), or a lot of other software. Search MacUpdate for similar software.

On backing up, if you have Leopard, you could make a partition for each Mac with 10.5 on the external drive and set it up as Time Machine.

I did that for my iMac and MacBook, it worked.

 

Getting two smaller HDs is easier for you to set up, as you won't have to partition the drives. If your computers are not close, it may be a little more convenient too, as you can just keep it close to the computer and not shuffle it back and forth. Also, if one fails, at least you only lose the backup of one computer. As for the external HD, are you going to build it yourself or buy them? Either way, I recommend Seagate drives. Very reliable, and the 5 year warranty can't be beat.