Randy,
I'm not too sure about "drivers", but I don't think most of today's monitors really need them unless you want to have system control over geometry and color.
I have a LaCie ElectronBlue that runs without any additional installing of drivers. Likewise, my ViewSonic at home is running on my G4 without any kind of drivers.
So I think as far as drivers are concerned your real worry is with the video card itself. I know that OS X has built in support of the latest nVidia cards, that's what is in the new G4's, and I imagine that since the laptops and the iMac use ATI cards, then there is built in support there as well (not to mention the fact that you can BTO with an ATI card).
ATI's PCI Radeon card is dropping in price, and may be just what you need. I'm not sure what the prices are on the nVidia cards, but they are usually pretty competitive. And it depends on what the second monitor is for. What kind of monitor do you want to hook up to it, etc. Before my recent upgrade to a G4, I was using a cheap ATI Orion 3D card to run my second monitor (which was only a 13"), because I used it mainly for palletes...so your video card will likely be determined by your use of the second monitor.
I think you'll be okay, as long as you double check the compatibility of the card with OSX...and make sure you check it's compatibility as a PCI card and not an AGP card. I don't think the type of monitor will matter, unless it's an Apple monitor...a new one with ADC will cause you some problems, you'd probably need an adapter, but if it's any other CRT or LCD I can't see you having any problems. Good luck, wish I could offer more help.
