Do you have two optical drives?
If so, it will be simple. Replace one of them with the 80gb disk and partition it as required. Copy data to it from the 40.
Format the 40 and create an image of the partion on the 20 which has the OS. Copy all other data from the 20 to one or other disk. Use PM to ensure partition is bootable.
Remove 20gb disk.
Install 40gb as master.
Install 80 as slave.
Replace optical drive.