it is a new one.
Without changing the partition table on the G5 drive, it will not be able to boot the Intel machine, but, it will be recognized by the MacPro and data on it can be removed.
Perhaps the easiest way to do the migration would be to use the Migration Assistant.
This will launch the first time you start your new Mac and ask if there is anything that you would like to move from an old machine.
Say yes and follow the instructions.
You will need a Firewire cable to connect the G5 to the MacPro. If your G5 has a Firewire 800 port, I believe it has, you will need a Firewire 800 to Firewire 800 cable.
Start the G5 in Target Mode, "T" key on startup, and then select your Account from the window that appears. All your network setting and other preferences will be set, all applications that will run will be transferred, all your music, document, photo's, movies will also be transferred.
At the end of the move, which can take a while, your new Mac Pro will have everything that your G5 had, right down to the desktop picture.
I'm thinking of moving from my trusty G5 PowerMac to a Mac Pro. I have a lot of things on there which i want to retain if at all possible as reinstalling everything would take literally days. Will there be any way of swapping the Leopard system drive over and updating it to work in an Intel machine?