This is all the bits you will need apart from the PC, "outside the box" as it were
Technically you can get away without any of these parts, but sooner or later you will want to upgrade...
Logitech Anywhere mouse mx.
You will need to use a mouse, and a wireless is the best for a media centre.
The new "darkfield" technology from Logitech works on glass (and other stuff also).
Microsoft ARC wireless Keyboard.
For some reason every decent wireless keyboard was either too big (why do they all have numeric pads?) or too expensive ($300 for a diNovo - really?).
In a stunningly rare example of picking the niche, Microsoft made the ARC keyboard which is small, functional and looks nice.
Amazingly, it can be found for as little as $50 on www.staticice.com.au (a meta price searcher in Australia).
Media Center(sic) Remote
Microsoft make a good one, although they are getting hard to find, there are many third party ones. Media Centre will suck without a remote.
Windows 7 Home premium 64bit OEM
No one really buys a new version of windows, but Microsoft goes out of their way to make an in place upgrade virtually impossible.
The OEM version costs the same as an upgrade, but can be installed in one go.
Yes Microsoft suck, but this is actually good software, and at the cost of slightly more than the mouse, you may as well buy it.
Home premium is the ideal version. Remember - the "Professional" version does not have Windows media centre.
64 bit is necessary if you ever want to use more than 4GB of RAM.
both 32 and 64bit versions run MCE equally well, there is some software that does not run so well on 64bit, but in the long run, 64bit is the best choice.