A NAS only sends data as fast as the network. Standard 100Mbit wireless network = 10 MB/sec or half the speed of USB. If you are going wireless to and from the NAS cut that in half. For streaming audio/video that's fine. For initially loading the NAS transferring a TB of data at 10Mb/sec takes a LONG LONG time.
A Gbit ethernet wired infrastructure supports up to to 100 MB/sec with good gear, 50 MB/sec with average.
My router lets me attach a USB drive and use it as a NAS device, but I've never tried. Seems bad to load down the router CPU with NAS functions.
You can also get a 6TB (3TB if you raid 1 it) 'personal cloud' lol NAS from Western digital. http://www.wdc.com/en/products/products.aspx?id=620 $420 from amazon, I haven't read reviews. I don't like all-in-ones because when something breaks you throw them away, however they are simple and cheap. Two of them at $840 gives you 12TB of disk (6TB if you raid1 them to protect against drive failure).