If you don't wanna game use a Linux distro. If your a power-user who's very competent I would highly reccomend Gentoo Linux.
If you want something easy to install and very easy to use, Ubuntu is going to be better.
They use less resources than XP and most certainly Vista, so the system will be much faster and responsive. Make a list of all the programs you're going to want to use on your computer and make sure they're linux compatible.
Plus these OS don't take up as much room as their Windows counterpart, saving you room on your 80gb harddrive. 80gb should last you a fairly long time, I have had my computer for over a year and have only used up 40gb of my 150 available (and I have a couple of legally ripped movies and a fairly extensive music library). Just get a secondary hdd later down the road when/if you do fill it up.