two monitors on one pc

I have two mintors the 19 inch is on my agp slot, and I want to add a additional 15 inch on a pci slot.When I install the pci montior wich one will be the primary?I want to use the AGP 19 inch as primary.Will this work?What are my options.I have winme, with a 800 mhz P3 386 ram.