Just to let you know what is possible. I am running 4 monitors on Win XP Pro with a G550 agp dual card and a G450 pci dual card in the same computer and it works flawlessly. Before I got the G450 I was running the G550 agp card with two Diamond Viper 330 8mb pci cards in a Windows 98 machine. The only problem I had in the Win98 machine is that one of my monitors operated at a lower refresh rate than the other three. When I would reboot all the monitors would be reset to that lower rate and I would have to manually go to the display settings and reset the other 3 to the higher refresh rate. I have now upgraded my 4th monitor when I got the new computer and don't have that problem because they all operate at 85hz.
Can you choose to have 3 of your 4 monitors act as one expanded screen, leaving your other monitor as "primary"? (I use MetaStock with 3 of my 4 moniors set up in MS to act as 1 screen, recognizing that is different from "expanded desktop". However, I'm running 4 single ATI cards.)