You need an interface between your network and the DS. This could be PSA’s own Bridge or a third-party box. The Bridge fits in the expansion slot in the DS, so you don’t need a separate box. But it does not support gapless playback and some people find it a little flaky in terms of keeping the connection reliably (and others find it works just fine; this depends largely on how robust your network is). There is Bridge II coming out by the end of the year that will solve the gapless problem and (we hope) be more bulletproof. Two third-party boxes are the Auralic Aries and the Sonore Rendu. I believe there are others also. I use the Bridge so I can’t speak about others from personal experience. With the Bridge, album art is displayed if it is available on the NAS; I am not sure about what happens with others. But when you use a control point on your tablet/smartphone, you would see the art there so the DS’s display may not matter (I leave mine turned off).
Your ReadyNas may have a media server app available. I run a Synology NAS, and Synology offers a media server called Audio Station. They also offer an iPad app to control Audio Station, although you can of course open a browser window on your iPad and log in to the NAS and control it directly. Or you can use any UPnP control point on a tablet or smartphone. Check out wherever you normally download extra stuff for your ReadyNas and see what they have.
If ReadyNAS does not offer anything, or if you don’t care for their software, then I strongly recommend MinimServer: very good sound quality, lots of flexibility in displaying tags, great support from the developer. They have an installation package for ReadyNas. You also need Java on your NAS. You would use any UPnP-compliant control point. Let me know if you have other questions.