Can I use my onboard video card for a third monitor?

I currently am using an NVIDIA GeForce 8400 GS PCI-E video card for my two monitors and am wanting to add a third. How can I do this? It seems that by using a PCI-E video card that my onboard video card is disabled or unable to be used simultaneously. What configuration can I use to have all three of my monitors work? I do have 2 AGP slots available & I am using Vista. Maybe I can use my onboard video card along with a different AGP video card? Let me know my options. Thanks