You need a copy of XP Home... The upgrade version of which retails for about $100, though supplies in stores are likely to be limited since Microsoft is trying desperately to push people to Vista.
You probably also want a CPU 500MHz or faster, with 512MB of RAM to get acceptable performance out of XP. Also keep in mind that there will be a few programs that will not run with XP that did run on Win98. They should be few and far between unless they're DOS apps.
how much does it cost to upgrade from windows 98 to windows xp? and what all is needed to do it? thanks for any help.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic