It really depends on what you want to do with it. If you want to be able to play with almost anything out there, the various Linux distributions are among your best bet. If you have a goal of getting most things out there to work with Windows ... well, good luck, and you obviously need Windows. Before someone thinks me facetious here, I do understand how some people feel a need to get things to work the way they work rather than getting themselves to work the way others work. That would explain why I stayed on OS/2 for so long ;-)

Personally, when I got my 1.8GHz/1GB RAM box, I slapped RH7.2 on it immediately. I then upgraded to RH7.3 pretty much right away. And then to RHEL3. Getting another box, I've slapped Gentoo on it. Interestingly enough, I had no qualms compiling new versions of perl on the RH system, but am finding myself a bit reluctant to go compile the latest perls on Gentoo. Wierd.

Anyway, I'm an anti-MS bigot (well, just anti-MS-crappy-software - some of their stuff, like MSWord or MSVC, are pretty good), so take my suggestions with significant portions of flavour-enhancers. For popularity that helps ensure most modules will work, I suggest Linux. And even there, I'm starting to see why Gentoo is a good choice - I've had much less in the way of problems in getting things like Gtk and Tk and GD and other such modules compiled on Gentoo than I did on RH.

Warning: it took my 3.2GHz, hyper-threaded, 4GB RAM box about 48 hours to compile KDE. So if you take my advice to go with Gentoo, count on it taking a while. ;-) All told, this machine probably spent about 100 hours compiling Linux to get the system running with everything, and probably has spent another 80-100 hours since then compiling upgrades. Of course, you don't need to upgrade if things are working, I'm just doing it because I can. ;-)