OEM is basically.. just the software/hardware. WIN XP PRO OEM would be just the installer and product key. This is opposed to the retail version which comes with manuals and a bunch of other stuff.
Its actually pro, with an extra gui for media, but the ability to join a Domain is removed in the GUI. Ya can get round this during windows setup though.
I didn't even know there was a new Media Center out. I remember the specs for the 04 and apparently, the MCE 2005 is much improved with HD support and the dual tuner support. I don't know about the Pro part. I'm still thinking it's based on Home Edition core but with a lot of tweaks to it.
I've found that media center had a lot of stability issues. I had it on my PC before I got a new HDD with Home Edition OEM and I had nothing but trouble with it. The media center liked to crash a lot and wouldn't load a lot of the time. I prefer home edition because it's cheaper and it seems to be more stable. Pro is good too but I have no use for all the extra stuff in it.