I’ll try the Ubuntu 10.04 on it when it comes out. Yes I see that it is free :-)
If it ends up seeming sluggish, which is possible given the computer's age, you could also try the xubuntu distribution - it is developed in parallel, but more suited to less-powerful systems.
Also, if you run into problems (though I hope you don't!), there is lots of information available on the Ubuntu Forums (www.ubuntuforums.org). Best of Luck!