Yea I'd say your XP on the 40gb.
And you Linux on the 20gb.
But I on the otherhand would recommend slackware, it's what I started out with and it was a piece of cake, esspecially with the help of this forum
I havn't tried those 2 distro's that syg00 said to try, but I figure those would hold your hand on everything you do.
So I recommend slack because once you get the basics down, your going to want a full fledged, balls to the wall Linux that you can keep around for a long time, and slack is a good solid distro.