Cool alternative to RAID...
http://www.drobo.com/
Keep adding any size drives or swap out the old small ones. Raid 5 like minimal overhead and similar reliability. Still need 3 drives to be worth while, but teh ability to add larger drives as they get cheap is cool. Looks like it is best to upgrade in pairs, but better than doing a whole array at a time. No, it is not cheap, but data protection ain't free.