This may get locked soon, but here goes.
I had a Popcorn Hour A200 until recently. It plays just about everything you can throw at it but wont decode to 6 channel PCM. Long term, this is a limitation for a Meridian owner looking to move to HDMI/HD621.
I believe the C200 does support this but havent had one myself. But then you are paying for a spinner as well and IMHO you would be better off going for an Oppo spinner so you can play Bluray/DVDA/SACD as well. Unfortunately although Oppo does support streaming of films and music, I found a number of films wouldnt play and they have no media wall front end which makes usability a major problem.
The Dune is probably better if you want a dedicated film streamer than the Popcorn Hour, I havent actually owned one but I believe they support 6 channel PCM decoding.
Given that you have the MacPro, you could use Plex as a front end to your movie collection. This works extremely well, which when you factor in the price makes it incredible. You can use Handbrake to rip DVD's.
Rather than spend money on a media streamer. I would stick to the MacPro, and instead spend the cash on PureMusic/HalideBridge combination which would give you a very high quality music streamer as well. You can then use Apple Remote to control iTunes for music playback, and the Plex client to control Plex for movies.
Thanks
Greg