Cheapest and simplest way, I believe, would be to find an inexpensive Firewire HD enclosure. As long as you ensure the enclosure takes an ATA drive (not an SATA), you should be good to go.

Then use SuperDuper! (or CarbonCopyCloner or a similar program) to clone the HD and make it bootable.

From there it's just a physical swap and you're done.