Problems with dual monitors

Hi all,

I'm trying to get dual monitors (two Mag Innovision 17") working. I'm
running Windows XP, and I have two video cards, a Nvidia GeForce4 MX 4000,
and a Nvidia GeForce4 MX 440. I have a Soyo P41 845PE Mobo with 512mb ram
and a P4 2.8C 800FSB processor, along with a 420 Thermaltake PSU. I'm
having a problem getting both monitors to work at the same time.
I'm postive that both cards and both monitors work. I take one of the
cards out, and try both monitors, and vice versa. I have my main vid card
in PCI slot 0, and I've tried the second card in all other slots. It
resides in slot 1 now. The problem is when I get the computer to boot
(sometimes it gets to a black screen, after POST, and after the Windows
splash screen, and just hangs, eventually rebooting.) I go to the device
manager and it recognizes both cards. I then go to the display property
settings, and I see an option to span to the other monitor. I check the
box, and I hit either OK, or Apply. Every time the computer freezes
immediately after clicking, and then reboots after a period of time (45
seconds??). Does anyone have an idea of how to fix this?

Thanks,

MC