Can you get the new computer (and firewire 800 external drive) without having to get rid of the old one?
Plug the old computer into the new one using firewire 400 and start it up holding the “T” key. Then, copy all the media to the new external drive and copy the project to the new computer and shut down the old computer.
Now, go nuts. It doesn’t matter if something breaks, you can always open up the old computer and get right back to work.
I know it’s tempting to sell an old computer, even if it’s only for a few hundred dollars, but hold off until the project’s done and you’ll have a nice backup sitting there in case anything goes wrong.
Zero risk, except for the loss of your time.