I use Fedora, which will work fine.
But everyone here will have an opinion. With that machine, just about any distribution will work. They all have their own pros and cons, and you just have to dive in.
Keep in mind you (probably) have YEARS of experience with Windows, so you know how to use it, update it, where to look when there are issues, install new software, any many more things like that. It will take time and effort to learn to do those things in Linux, and those are the things that are different between distributions.
Take a look at ThinkWiki's page for that model:
http://thinkwiki.org/wiki/Category:X20
They have specific instructions for Gentoo (2005) and Debian, but other distributions will most certainly work too.
Let us know what you choose!