What filesystem is currently used for Win2K? If it is NTFS, yer gonna need the latest Ghost (6.5). I am not sure if it will let you ghost to a smaller partition (shouldn't be that big of a deal). In either case, you *may* want to create the partitions on the 20GB HD in advance.
When you get the new drive, put in on Primary Master and move the 18GB (assuming that it is already PriMas). Ghost the WIn2K to 10GB or whatever. Disconnect the 18GB and try to boot Win2K. If it works, you can then go ahead and reconnect the other drive and make other partitions.
before installing Win9x, make an emergence repair disk with the backup util. When you install Win9x, use 'setup /ntldr' and hope that it won't overwrite the NT loader. It *should* just add a value for Win9x on the menu (you probably haven't seen the menu before because only WIn2K was isntalled; but it's there). If the Win9x install doesn't play fair, you'll need the Win2K CD (and your repair floppy) to repair the boot sector.
-SUO