For Windows XP, you can install different graphics cards and different graphics driver versions and run them at the same time. So you could use two NVIDIA or two ATI, or one of each.
For Windows Vista, the WDDM restricts you from only being able to install a single graphics driver from a single vendor. So you would have to use two ATI or NVIDIA graphics cards and they would need to be supported in the same driver. You can read more here http://www.microsoft.com/whdc/device/di ... Vista.mspx
The other thing to simply be aware of is that most game try to use the monitor assigned as the primary one for running 3D games on. So if you want to use multiple displays, make sure the iz3D one is marked as "primary" from the windows display properties control panel.