I have run 3 monitors for the past 12 years on all my Macs. I just retired a Dual 2.0-Gig G5 that had 3 monitors connected for the past 4 years. I now have a new Mac Pro running 3 Sony 23" CRTs.
What I always do is hook my Main monitor to the card in the fastest (lowest) slot and then buy a second PCI card, put it in slot 2 and hook up the other two monitors to it. That gives me full speed to the main monitor that may be used for video often. I had an ATI 1900 in my latest G5 as a second card, but you will just need to see what is compatible with your particular machine.