It's all done, and has been for a little while.
Everything that could go wrong, did go wrong.
I'll probably have to reformat and reinstall at some point, but not now.
I would have then if I'd bothered backing up
It runs, that is enough for now.
Total changes:
Went from a Tbird 1GHz to an Athlon XP 1800+
Went from 512MB PC133 ram to 512MB DDR2100 ram
Went from 10base 3com NIC to 100base onboard
Went from old 250W power supply to new 250W power supply (yup, old one broke when I touched it, err, smacked it really hard)