Multi monitors (1 AGP and 1 PCI) can cause stability problems. However they can also work very well. There are some good resources on the net.
Check out http://www.realtimesoft.com/multimon/
I have installed a couple of different computers with multi monitors (for my parents and a couple of friends) I have two suggestions when selecting hardware. Try to buy both cards from the same chipset provider. (ie nvidia) and avoid ATI at all costs. I have a couple of systems that I have tryed getting an ATI card to be the secondary controller and have repeatedly failed. If both cards are from the same manufacturer my theory is that they have at least put two of their cards in the same PC and tested it.
Both the nvidia (geforce 4) and matrox (G550) have dual head cards available. I would at least consider that option as well.
