OK, simply put a NAS drive is that a (N) Network (A) Attached (S) storage deivce so it doesn't require anything in the pc, it's normally a standalone box that has 1 or more HDD's in which you link into your wired or wireless network.
For what you're looking to do you WILL need more than a 2Gig setup, i'd think a minimum of 2TB (Terabyte) and i'd sugggest for backup / data recovery you'd be best with a multiple drive system, either 2 drives mirrored (meaning the same data is saved on both discs) or a 4 drive RAID system (where the system is written in a special way that the data can be recovered in a faulty drive situation)
It all comes down to cost and what you want, have a look on Ebuyer / Overclockers at NAS system by Buffalo, Synergy, Linksys or Netgear and as you say, look fr one that has DNLA or media Streaming capability
Hope that helps, if you need any more info lemme know and ill try to reply before we leave the US, if not we back next fri so can answer then