All Windows Service pacs are free for at least five years, and so far it's going on seven years for Windows 2000. XP is not a significant upgrade for a 2000 user like me. XP will be supported free for several more years.
XP users will be able to upgrade to 64 bit windows at no cost. So far AMD is the only 64 bit chip, but Intel will have one out this year. Professional applications like Photoshop and music recording programs like Pro Tools have 64 bit versions.
To upgrade to XP from Windows 98 costs about $90 if you shop around.