I found it cheaper to buy the housing and HDD seperate.
I got a USB 2 'Icy Box' (there are other brands avalible), and put a 120Gb Seagate in it, works like a treat. Its just not as portable as I'd like as its not USB powered (it has a seperate power pack).
Still, it only sits next to my PS2 for running 'backed-up' games
I have a freecom classic network HD 160gb Just plug it into the router all my computers on the network can read and write to it, also if you want to add more you can set up a raid system. Raid 0, raid 1, raid 2 or even raid 4. I paid £129 from pcworld but I expect if you hunt about you may get cheaper