If you are looking to stream to your Roku, you might want to look at Plex. It can run on some of the higher end NAS. However streaming HD video can be a bit CPU intensive so you are better off using a PC for your Plex server.

I have a NAS, as my fileserver, and an older PC running the Plex server. The PC is dedicated to only running the Plex server, and my media is stored on the NAS. The NAS is connected via a Gigabit connection on the router and I have no issues streaming. The NAS backs up all the computers in the house and everyone has access to it on the network. If the NAS ever gets full I could put another hard drive on the Plex server computer and move media directly to it.