I too have been looking for a similar solutions. I was thinking about getting something like this: http://www.ubid.com/actn/opn/getpage.asp?AuctionId=6495561
Quantum Snap Server. This way all of the data on it could be available to any computer on my network. This would be very nice for sharing mp3's to my AudioTron network mp3 player. Currently my main computer has to be on to use it.

Even thought this particular one only has 120GB, it consist of 4 30 GB ide drives. I read somewhere (I can't find it now), that you can easily replace the drives with larger drives. If you did 4x120, you should be set for a while.