You cannot use a dual monitor setup with both the onboard video and an AGP card. The AGP bus is designed to handle only one device, and cannot use two at the same time. The onboard i752 is an AGP device.....so if you want to go dual monitor, you would have to do it with a PCI video card.
Both the i815E and i815 are subject to this limitation.....actually, it's an AGP specification problem rather than chipset.