format, go xp pro
i like 2k a lot, its much better then 98. and it seems to take up less space, use less memory, and perform better then xp. the only issue is sometimes it doesnt support games, and i found myself wasting space on a dual 2k/xp boot, so i ended up saying screw it and just putting xp pro on. theres probably a lot of bloat you can cut out of xp, if you are concerned about that sort of thing for performance reasons.
but definitely, absolutely do not stick with your current installation of 98. when you install a new mainboard in a 98 system, sometimes it works out ok, but many times the os will freak out and crash even if you've installed the drivers for it. so you may be looking at an os reinstall anyway. plus, remember lots of slowdowns are caused by software bloat. so reinstalling is always good..