Thanks everyone for the info and also the comparison site. The online backup decision is going to take some additional study. Backup is a more complex issue than it seems at first.

Some files, like family photos, are irrepaceable. Some are updated often, like my budget spreadsheet. Others are created once, then stored unchanged forever, like tax returns.

I'm planning on using the NAS mainly for shared media storage (mp3's & movies). These files are not really irreplaceable.

I see that Carbonite, for instance, has unlimited backup but only for files that actually reside on your local hard drive. Some services charge by the Gig. Either way doesn't seem practical to have online backup for everything.

Ok, I think I'll get two 2Tb drives for the NAS and configure them as RAID 1 for some bonus redundancy. I'll store or at least keep a copy of all irreplaceable files on a PC and protect them with online backup service.