Home entertainment PC's

[Bifta]

I want to build a small, either shuttle sized or mini itx sized PC, I don't know a great deal about them so I'm hoping someone knows a bit, what I need it to do is:

Record & Pause live TV
Play DVD's through the TV via scart (probably with an svideo - scart adapter than natively via scart)
Run Windows Media Centre Thingy
(I've got an MCE remote already)
Play MP3's via a seperate amp
Preferably be wireless, but I can use a USB adapter for that

I have some hard drives for it already .. but that's about it, I'd probably invest in another cordless keyboard/mouse (Logitech).

Anyone have any suggestions, or cheap places to get hold of the right stuff?