Depends on how well your current computer suits your needs, etc. I've got a nine year old laptop that's still quite serviceable for some tasks -- it'd be even moreso if it had a functional CD-ROM (wore out). I've also got a three year old desktop that is only slightly less-powerful than some of the low-end Pentiums you can pick up today, though.
Honestly, 18 months is an eternity in the computer world. I wouldn't really let it be much of a factor in my decision. The $100-300 to upgrade in that time is insignificant compared to the amount of increased productivity you'll have until then. Besides, you've already demonstrated a willingness to work with older tech, so it's not like you need to upgrade next year. Also, Win XP is a whole lot better place to rest, IMO, than Win 98.
FWIW, I'm currently waiting for FedEx to deliver a new computer to me. The only factor OS played in my decision was to make sure I was getting XP Pro vs. XP Home, and even that's because I like gizmos more than need.