If its a 1998 era motherboard, then chances are it has at least a couple of PCI slots.
There are still some companies that sell PCI video cards.
Even if your motherboard has an ISA slot, just stay away from it and use PCI.
Also, with most Compaq desktops with built-in video adapters, you have to do a little dance with disabling the built-in video adapter and telling it that you are going to use an add-on video card.
Depending on the motherboard they used, the BIOS setup could either be a breeze or a nightmare.