The other option is to install the Plex Server. You download this from plex.tv. It supports a wide range of formats, particularly if you enable transcoding. It has it's own player, but also has it's own DLNA server capability - which will work with a wide range of DLNA players. Plex has it's own extensive forum (as well as knowledge base articles).
If your player can play directly from a network share, then of course it will.
ReadyDLNA will not play it.
Plex will play it (at least it played the sample I found), but I believe it has to transcode it. In general it'd be best to get a RN500 or RN600 series for this. However, the RN21x and the RN426 or RN428 likely will work ok. Note the RN422 and RN424 have dual core processeors, the RN426 and RN428 have quad-core processors. I haven't seen anything posted here about the transcoding performance of the dual-core models, and since I don't own an RN42x NAS I have no direct experience.
I suggest installing the plex server on your PC (windows or mac) and see if it will play the content you have - my own media library doesn't include rmvb, so my response here is based on checking with one test sample.
BTW if you are ok with leaving a PC on 24x7, you can just keep running plex (or any other streaming server) on the PC, but store the media files on the ReadyNAS. That approach gives you the most flexibility.
