Changing Graphics Card on a 2006 Mac Pro
If you have a random low end PCI express graphics card you could probably put in any card and have it work and the most basic level. Guessing it should just work with no drivers on OS X although  it will probably be a horrible resolution and have no graphics acceleration, but it should work for when and if you need to throw a monitor on the Mac Pro when running OS X. However when booted into Windows it should be able to pick up the drivers from Windows Update or you can simply download and install drivers manually for any Windows compatible PCI Express graphics card.