On a Mac, how would I use a CRT for monitoring?

Can I send HD back out Firewire or do I need some kind of card for it to work?

Software-only solutions do not offer hdv out through firewire for preview purposes - in fact, even Canopus NX can't, although it comes bundled with a HD graphics card offering component out to monitor. I'm afraid for Mac you'll have to spend $ 600 on Decklink HD for that luxury.