@GTX:
I can just get a 2.5 IDE drive and put it in without any adapters?
Sorry for the delay. Most of those boxes come with an IDE connector suitable for connecting a laptop style drive in a caddy. You will need an adapter to connect between the actual 2.5" IDE 44pin connector and the connector on the board.
It's mentioned here: https://doc.pfsense.org/index.php/PfSense_on_Watchguard_Firebox#Further_Enhancements_3
Here's an example: http://www.ebay.co.uk/itm/253003464107
As I said before you definitely don't want to spend too much here because the 32bit hardware in that box has limited life at this point.
Thanks,
Steve