"Can I just swap the hard disks and reboot using the recovery disk or will I have to get a new 40GB HDD."
Unlikely Baffer,due to the possibility of the recovery discs needs access to a possible hidden partition that contains the os.
You may need to buy a new version of windows for the other drive.
You could try using Acronis and clone the current drive while windows is working ok,and transfer the clone to the new drive (Hitachi 30GB DK23DA-30F)
You will need a usb adaptor or caddy for the Hitachi drive temporarily.