Overall, I have been very happy with XP. It is the best version of Windows, ever. Win2000 (NT 5.0) was a good step forward, but in Microsoft tradition, XP (NT 5.1) is more stable and versatile.
That said, and now with Vista out there, I'm thinking about the future and installed Ubuntu on an old box (Pentium II 400 mhz, 319 or so Mb of RAM) I have. So far it looks pretty good. Ubuntu is very intuitive and handles some network tasks easier than does XP. My other boxes and laptops we use regularly are Pentium IV 1.5 ghz or better with at least 500 mb of RAM. There may be some compatibility issues with hardware,but I think I know where I'm going when I have to upgrade.
In my opinion, Linux is the best available OS for servers, but it's got a long way to go for desktop usage. The configuration tools and user interface standards are a mess. I wish that situation was better, but there is too much chaos in the Linux developer community to deliver a consistent, high-quality desktop OS.