at my work we have two NAS boxes for this, a primary and secondary which is a live sync of the first. I didn't have anything to do with the setting up of the boxes but I know that they're not running windows (I think they're kinda indepdant and run their own OS). Because the data is historic read only stuff we don't do daily/weekly backups or anything the only backup is the second server. We can access just as a PC share so to the user there's no difference to files on the NAS as files on the normal (windows based) file servers. As for capacity I think the sky (and your budget) is the limit.