Why not just migrate the site to the latest version of Xoops? That way you should be able to retain the look and content of the existing site better than trying to migrate the whole site is a completely new CMS model.
Drupal is a very nice CMS. It is one of the most versatile, but also one of the hardest to get to grips with. It is also mainly a framework more than offering a complete site structure out of the box, meaning the learning curve is quite high.
An alternative if you don't want to stick with Xoops is Joomla. I've been using it since the Mambo days and it is my preferred CMS over all others. And with Joomla 1.5 the CMS's core code is better than ever, more streamlined and easier to work with. And the mod community for Joomla is bigger than any other CMS on the market, commercial or open source.
One question that springs to mind. What are you going to do regarding forum software? Have you looked into how Drupal bridges the membership accounts with the forum software you are thinking of using? And also if the existing members accounts can be ported?
