The Matrox looks like a nice card - but a little more expensive than I have to spend right now - plus there's no guarantee it will "play nice" with my quadro card...

No idea yet. Been meaning to test it on another machine for weeks now. I have an RMA pending with Black Magic.

Ditto on this. This is a home/hobbyist setup and the Matrox is way out of budget. (Though my hardware guy at work, a former Avid editor-turned hardware evangelist), says run, don't walk, from Matrox products...)

Do I really need an I/O Card?
 

I'm looking at a new Mac Pro and want to run two lcd monitors plus my Vizio 1080p plasma lcd simultaneously. I'm cutting XDCAM HD footage and will be doing green screen compositing at times. The few people I've asked have said I must get a BM Intensity Pro or Kona LHe or Matrox MXO or some equivalent i/o capture card in order to run the video to the plasma, but I don't understand why I can't just add another video card alongside the upgraded ATI Radeon HD 4870 card that's offered with the new mac. I'll be adding a Sonnet e4p card, but that should still leave me enuf space, no? For that matter, couldn't I even use the S-Video out on the Radeon to component?

I know it's not a broadcast quality color space, but I'm not so concerned about that. I've got an old Sony NTSC monitor that maybe I could hook up to it to give me good color when it's time to do color correction, albeit in SD.

The Vizio currently gives an excellent HD picture out of my MBP using a dvi/hdmi dongle, but because of the limited number of slots I have to change it out each time I want to go back to the desktop monitors.

The difference between an Lhe with breakout box and a decent vid card is over $1500.

Does it have more to do with computer processing than graphics processing? What am I missing?