Sooooo Many choices.....
A PS3 or Xbox360 can stream from a UPNP source (bot sure if your NAS has a UPNP server). Failing that your best bet (IMO) is to get an Apple TV or a nettop PC and install XBMC (it will install on either, check youtube for a how to on Apple TV). XBMC plays back most file types and will check with online databases to download DVD thumbnails for all your files.