Capture Card for grabbing screenshots?

I have a video processor device that hooks into the hdmi chain right before the display, and i'd like to get comparison screenshots of the the video being sent to my display when the device is "On" vs "Off". Here's the device i'm referring to... DarbeeVision Darblet. I want to take 1080p PNG screenshots that are 100% accurate representation of what i'm seeing on my display, so i guess that would mean uncompressed. Can this be accomplished with some kind of capture card, and what would be the least expensive way of doing it... or, what's the least expensive capture card that could do it?

If you have a desktop (Mac or PC) cheapest would be an Intensity Pro Card. (approx $190)

That will allow you to use the supplied Media Express software to capture screen shots or video in uncompressed.

Check the specs to make sure the video format you are using is supported.
http://www.blackmagicdesign.com/product ... techspecs/