Help needed in choosing video production device for use with Matterhorn Capture Agent
One of the best resources you will find for this is probably the Matterhorn documentation itself. The Capture Agent Hardware page has samples of hardware that was used at a number of schools that have implemented capture agents for Matterhorn.  It appears that the Epiphan VGA2USB is the near universal standard for VGA capture.  There seems to be a wider range of video capture devices.  

Personally, I'd probably take a look at seeing if the Black Magic Intensity or Matrox MXO2 works with the capture agent, but there are also other listed devices there such as capture devices from Hauppauge or Bluecherry which have actually been used successfully already.