It's not quite that simple:
1. The drive must be IDE, but not SATA
2. The maximum supported size is 120GB
3. You need a Network Adapter
4. Western Digital drives are a no, no.
5. Many drives also either don't fit or don't work. You should check compatibility with this site:
http://ps2drives.x-pec.com
EDIT: You beat me to it, Eakinsme,