Its easy...
if you have an AGP board ( i assume so!)
just drop in a second vid card of the PCI persuasion and boot...
hook your 2nd monitor to the PCI card, and try it out, if the BIOS tries to use the PCI card as the main card, get into your BIOS and change it so that it seeks AGP as main card...
once you get the right monitor set as the main one, go into setttings/display and enable the 2nd monitor and set all the parameters you want...i.e. resolution, colors etc...
i have 2 monitors on 98se right now and i even have a 3rd PCI vid card in there but i bought it second hand w/o drivers so i cant test out the 3 monitor theory yet...but i doubt it works...
hope this helped
Luke