You can never have enough storage!!
My NAS has 5x2TB HDDs - gives me around 7.25TB useable space.
The My Book Studio you can see on the right is connected to the NAS via eSATA. Its the 4TB version. At the moment its plenty enough to backup everything, but if space becomes an issue I'll use it to just backup my most important data. eSATA is super fast!!
I then have 2x640GB WD Studio Passport drives (can just see them to the right bottom). I use one for iMovie video editing, and the other is another backup of my kids photos etc which i usually keep offsite.