I'm no Apple buff but I'll attempt to answer your question and then be prepared to be corrected by those in the know later on
Most NAS drives come with the ability to act as iTunes servers (AFAIK). I think you just need to make sure that you point iTunes to the place where the music is stored. This might mean transferring the music to a new folder on the NAS drive other wise you'll probably need to keep your PC switched on all the time so that the iPad and iPod can access the relevant libraries.
There is also the 'Home Share' option as well, if your NAS setup isn't capable of running an iTunes server.
Might be worth popping along to the Apple forums though - if you get no further responses here.
God luck and let us know how you get on. The wife was given an iPad from work a month or so ago and given the amount of wireless devices now in the house, I'm seriously considering a NAS drive, so any feedback from your experience would be greatly appreciated this end as well