What OS are ya going to be using? i don't have one for sale, but i can say that i would get one that is at least 40 gigs. anything smaller is going to lag in system responce time, unless you have a fast processor or alot of ram.
All i'm saying is that a 20 to 30 gig hhd will fill up fast, and would slow down as you added more software to it. Especially if you plan on running windows xp.