Can I hot-plug a DVI monitor in a Mac Pro? I have a Mac Pro with a DVI display, and I got a new DVI display for it.

I want to unplug the (only) display, and plug in the new display to that DVI port.  And/or, unplug the existing display, and plug it into a DVI port on another (slower) graphics card.  (Does that make sense?)

I've hot-swapped DVI displays on Mac laptops for years, but I've never done it with a Mac desktop.  Will a Mac be cool with temporarily having zero displays?
Absolutely!  At work we have multiple Mac Pro's that have 1 monitor between them hot-swapped and while there's up to 5 seconds of delay for the video to come through, it works great.

You can hot-swap displays, but not video cards - if that makes sense.