USB versus NAS devices?

I'm a big believer in backups.  Multiple forms.  USB drives, network attached storage, DVD (dual layer), etc.

As I accumulate more photos, my storage needs have grown.

Currently, I have the following:

• 80 gig USB - 5 gig free
• 60 gig USB - 15 gig free
• 160 gig USB - I *think* 15 gig free

At some point in the future, I'd like to get a new device, probably 500- 1TB in size.

I'd like to use this as my main offline storage facility for my photos. 

I'll still continue to do DVDs as well, but will hopefully be able to purchase a copy of Adobe Lightroom for use with this external device for archival/digital asset management use.

I'm torn, at the moment, between a device that plugs into my LAN (and therefore always on) or a USB/Firewire device that only gets turned on when I need it.

Thoughts?

Technorati tags: NAS, USB Storage, Archival