No display switched graphics card

So at work I've used two different towers--a 2007 Mac Pro, and lately a 2009 Mac Pro tower. I made the switch because the 07, while generally decently fast, was having some harddrive errors and making me a bit nervous about how long it would last.

The ideal swap was to move my user to the 2009 tower, which has better specs in most everything BUT graphics card--and then swap the graphics cards. The 2007 had a ATI Radeon x1920 xt and the 2009 came with a Nvidia GeForce GT120.

My coworker and I swapped the graphics cards and put the Radeon in the 2009 and the Nvidia in the 2007 and booted both machines--but neither had a display. The machines booted up, just... black, no display connection at all. We switched back to make sure we hadn't broken anything--worked just fine.

The 2009 machine already had drivers for both brands--Nvidia and ATI. I tried researching online but the general consensus among the Apple crowd is that the machine should already have the drivers, and at least display should be a given?

Seems like this is a simple problem! I must be missing a really basic step, simply because I've not done it before, don't know the process, and can't find a specific tutorial online.

Also, the Radeon required an additional power source, but it was plugged in. I can't think of anything to troubleshoot beyond this, I really don't know enough.