3
If at all possible you will want to copy your site to a test host and try the upgrade there first. Chances are you will be able to update your xoops core without much problem but some of your modules may not work right. Depending on the modules you use you may find replacement modules for the ones you already have.
If you need to keep your old user base then you might be better off removing all the old modules (Or atleast disable them) and then upgrade. An upgrade from 2.0.18 all the way to 2.5.1a might be a reach.
Good luck!
Rodney
