When my brother switched from 98se to XP Pro with his T-Bird it was very noticable. He increased enough fps to bump up some quality settings and still end up with 40-50fps in MoHAA. Not sure bout MCE but I would defininetly go XP.
Media Center Edition is pretty much exactly the same as XP Home/Pro... just with a few extra multimedia 'features'. They may be useful for someone setting up their PC as part of their home theater... but not really worth it otherwise.
<font color=red> If you design software that is fool-proof, only a fool will want to use it. </font color=red>