depends on what you want to do
if your pc is slow, then get xp
if its fast, you may want vista, but its up to you
vista is more stable and secure, but vista is incompatible with some peoples programs and some hadrware vendors havent released drivers compatible with it yet
however vista is a big change from xp, so there is a learning curve
if you do get xp, remember your upgrade paths
xp home -> vista home premium or basic but never ever ever get vista basic its shit
xp pro -> only vista business or ultimate
i dont know about xp MCE.
gamingwise, you may want 64 bit in which case, get vista. XP 64 is crap, whereas vista 64 is good
vista also has directx10 for newer games
but vista wil mean you will have less ram free for games than on xp - NEVER buy a vista machine with less than 2gb ram. 3/4gb (or even more if you want to take advantage of 64 bit) is better