If you use a second hard drive for OS X (which I highly recommend) you won't have to back up XP. The drive that holds Windows XP will not be touched by the OS X installer or anything else. It's perfectly safe, Disk Utility cannot write to NTFS formatted drives. The only thing you can do from Disk Utility is to format the entire drive, but that's not really something you can do by accident.
You could probably install OS X onto the external hard drive, I guess it could work. It depends how well USB (or your TI Firewire if it's a Firewire drive) works from the get go.
Actually, you could probably even install Snow Leopard on the external drive while it's connected to your MacBook Pro, assuming it allows that.
Then make the required modifications when the installation is done (Chameleon bootloader, configuration settings, drivers, kernelpatcher module or patched kernel) and then connect it to your PC and see if it boots.
That's definitely the approach I would take, and what I would have recommended from the beginning if I had known that you have an external drive.