Is it windows 98se or 98 and do you have any usb ports.
If it is 98se and has usb. It is likely the usb ports are usb 1.1 and not the slightly faster usb2.0. If so you should be able to use an external hard drive, but transfer rates will be slower.
If it is windows 98, you may have a problem. I can not think of any external hard drive that run off that system. All I have seen need windows 98se.
You do have another option for an internal slave drive. If you fit a pci controller card ( about £10 ). The on-board bios will allow your system to use extremely large hard drives. Drives in excess of 120gig can be used.