|mmm As per riider: an ethernet card that fits the slots in the '98 system; and has a cat-5/ethernet port to connect directly to the router...|
Or a wifi card. The latter (ideally) would be one that fits into a slot within the '98 case and has an antenna on its back plate; or usb adapter type...
Personally I've seldom found usb-wifi adapters much kop... But no doubt some pholks have them OK.
However you may find getting one (usb type) that has required drivers for '98 (if they're needed) a problem; unless you can locate a usb-wifi adapter that includes' '98 in its specs.?