I have a "Western Digital 80GB 7200RPM 8MB Cache Serial ATA" harddrive and want to upgrade (INTERNAL HARDDRIVE?

You will need a SATA HARD DISK of whatever capacity you want. Just remember to say SATA...open up the hard disk and measure its width. The sizes come as 3.5 and 2.5 and you probably need the 2.5 one.