Well, I don't really watch much TV at the moment on the big TV, but I want to use the media centre PC for watching streaming TV from TVUnetworks, SOPCast, and others on it too.
Generally I watch regular TV on my main PC and use one of my three monitors on my main PC for TV watching while surfing websites, etc... on the other two.
I think I would use my main PC for holding all the mp3s, movies, and other content and stream audio and video from it. As far as recording goes, I have a Leadtek PVR2000 TV Tuner with hardware chips for encoding and decoding, but that will probably stay in my main PC as I'm more likely to have that PC on for when the show airs that I want to record.
I bought a Cooler Master CM-282 case for the media centre PC so it has a remote for controlling the O/S functions, however, I'm considering getting a wireless keyboard & mouse. I have never played with XP Media Centre, so I was curious about it.
As for the media player... I have always used Winamp and VLC media player for any machine. I only use Windows Media Player if I have to. I have a BeyondTV account and they provide a basic media interface and player too. I have tried it, but would like something more feature rich with a polished look.