Great topic.
I love to build and I love to play, but this whole hobby is more about playing than building for me. My builds are designed around play, inspired by a desire to play, fundamental decisions about the project are dictated by how they will affect its play, etc.
Great advice above from miles2912 - use the PC in there to its full potential. My cab is first and foremost about playing classic coin-op games in an authentic way, but is also plays PC games, I have my Xbox360 integrated in it, it houses all my mp3s and family photos, it is also an email/web browsing workstation.
Eric.