If you have to get a HD for it, consider the OWC solid state drive - a 30 gb one is around $60. That's my next Pismo purchase, as soon as one of my drives fails...

Or, on the cheap side, Tony ZZ on eBay usually has 10gb used drives for around $20 or so...I have had good service from him for a wide variety of Pismo parts. Of course, he also has RAM if you need that too.