If your Windows 98 PC works fine, then why change it?
Re-installing or upgrading of Windows takes a lot of work and time, not to mention you must obtain all the required drivers and softwares. Also, you have to make sure your PC is powerful enough to run the new Windows.
Now back to your question. Well, actually there's no definite answer to it. It really depends on the config and usage of your PC. In generally,
Windows 98SE - occupies little disk space, can be easily run by slower PCs, quite stable, but doesn't support some of the latest devices Windows ME - The worst Windows version ever released by Micro$oft, quite unstable, not recommended Windows 2000 - runs fast, very stable, supports multi-language, good power management feature which is very suitable for notebooks Windows XP Pro - same as 2000 but bigger and slower and has some additional features Windows XP Home - as stable as XP Pro but with less features