I remember a while back I wanted three monitors and added a PCI video card to my system (3Ghz P4, 2G ram etc). I eventually swapped it back to the AGP card only which gave me two outputs. As I remember it the system throttles the graphics back to the slowest card (in this case the PCI). It was a very noticable change.
As I say, this was a while back and newer cards may be faster. Can somebody confirm this or tell me I'm talking mince