I don't believe it!

After all the palava, turn out FLASHING THE ASUS BIOS (via floppy) did the trick! All now perfectly fine running as it was with the old mobo - and only with a Repair Install. Bootiful.

I'll never trust an in-Windows-BIOS-updater again. I used this before I transferred the floppy to the new case. Noticed an odd "BIOS update data incorrect" message at boot that didn't stay on the screen for long, so thought sod it will do it the old way, and problem sorted.

BTW, the SLK3700BQE PSU is NOT quiet! At least not mine.