Hello,
The Operating System doesn't need to be on the 'master' hard drive in order to be able to boot into it. Master and slave are just terms describing the physical location of the hard drive on the IDE cable which it is connected to.
So for example, I could have two hard drives in my computer. Hard drive 1 is the master which stores only personal data files. Hard drive 2 is the slave which contains the operating system. I can still boot into the operating system on hard drive 2, even though it is the 'slave' hard drive. The boot order setting in the BIOS determines which hard drive is booted into first, and this can be configured.
As already said, you could just simply 'clone' all the data from your 80GB hard drive to your 320GB hard drive, then format the 80GB hard drive and use it to store backups of your files, and boot into the OS on the 320GB hard drive.
Alternatively you could start fresh by setting your 320GB hard drive as the primary boot device (in the BIOS) and install the operating system on your new 320GB hard drive. You'd then have your 80GB hard drive with still your data files stored on it, and of course with Windows still installed on the 80GB hard drive (which you can get rid of afterwards).