I think your server box is fine. Although 1TB of disk space is a bit on the small side if you are going to archive any HD
Not sure about the PS3 or Xbox 360 for front-ends, to myth anyways, but they can play the mythtv mpeg files. The mac-mini should also work, I used OSX 10.4 with myth (.20.2) since a leopard build had not been done, much less .21 version.
There are some really inexpensive options for playback esp. the nvidia all-in-one boards. I am using the asus pundit on one of my systems and that is really a nice setup.

There are some really inexpensive options for playback esp. the nvidia all-in-one boards. I am using the asus pundit on one of my systems and that is really a nice setup.