Bear in mind that 2003 is designed as a server OS, and so you may run into all sorts of problems trying to run general programs on it (for example it might not have complete DirectX support). If you want a machine to run a Windows-based web server, or you want to experiment with Windows networking, then go right ahead. If you want to play Half-life, stay away from it.
XP MCE would be a good choice if you wanted to use it *as a media centre*, or again if you wanted to experiment/learn. But again, if all you're after is a gaming rig then straight XP is the best choice.