Be sure you purchase video cards that are on Microsoft's list of supported cards or, better yet, get a card that already has dual monitor support. Most new systems on the market come equipped with a single AGP slot and several PCI slots. If you system is equipped with AGP, simply install a PCI card. However, before you can configure that card, check to see how your system BIOS controls video. You will need to check the BIOS in order to make sure the AGP slot is selected as primary.
Chances are it already is, but check anyway.
In this way the PCI card will automatically be configured as secondary. Make sure your secondary monitor is connected to this card. In the event you don't have an AGP slot, then you will use two PCI cards. Turn the system on and Windows 98 will recognize the new card along with the attached monitor and install drivers for both. When prompted, restart the system.