Your KVM switch needs to be able to handle the Keyboard connections to both the Mac and the PC. The Mac uses a USB connection, exclusively, for keyboard and mouse. The W98 Machine probably has a PS2 connector (Round plug)for the Keyboard and Mouse. Hopefully,it also has USB ports. If it doesn't you will have to use a USB to PS2 connectors, one for keyboard and one for the mouse, from the point where the KVM switch cables connect to the PC.
Having 4 USB ports on the Monitor does not help you a great deal, on the PC system, if the PC system does not have any USB ports on the case. The monitor has to connect to the computers USBus in order to function.
Belkin sell a USB KVM switch, F1DK102U, which is being sold by PCMall for $50.00. You would not have to purchase any other cables for this setup.
The Mac USB keyboard plugs into the switch and the attached cables go to the USB ports of the PC, or to the PS2 ports with the adapter, and the Mac.
Any USB keyboard and 2/3/4/5/6 button USB mouse will do alright for you. Logitech make good mice. If you buy the Mini from a mailorder catalog, MacMall etc, they will probably through in a keyboard and mouse for free. Ask about a Free memory upgrade too. OS X has multibutton mouse compatibility built into it.
Hope this helps