Much depends on how 'secure' you want your data to be. Part of the problem with the single drive solution is 'what if it fails'. All your data is gone. Yes, it is the -simplest- solution, but as with any hard drive it could fail and then you're hosed.
So - your next solution would be a NAS unit and of those, you have a few choices:
1) COTS (commercial off the shelf NAS) like Synology, Netgear ReadyNAS, LaCie etc or even some of the Western Digital NAS solutions (which offer multiple disks and RAID redundancy).
Advantages: dead simple, plug in the drives and it just works simplicity. Disadvantages: Cost, proprietary systems.
2) Nas4Free/FreeNAS/Linux box. Have a spare computer lying around? Plug in some drives and connect it up into the network. Use those for file storage (this is actually what I do).
Advantages: General purpose hardware, easily expandable, upgradable, and low cost. Disadvantages: learning curve for FreeNAS/Nas4Free/Linux. Sometimes have to get your hands a little dirty and have some technical knowledge and be comfortable with a linux system.