Hi Neil,
Sorry but this is a shamless plug for what I've been suing for years and is free. Knoppmyth. Linux based - so initially some fiddling underthe hood to get everything working as you want. But if you want a pc that sits there and does what a media centre does, it'd be the one I'd go for. (Shameless bias!)
Sure, perhaps not quite as pretty as Windows, but also a lot less horsepower needed. I pointed someone else in it's direction on here just the other day. The kille app for me is MythWelcome - it turns the pc on to record what I want and turns itself off again afterwards. It's easy to copy stuff over on the network via samba from my ubuntu lappy I just download, and drag and copy across. The tricky bit is usually setting up the remote control, but I believe with a usb irreciever it's quite straight forward.
Also, with the new nvidia drivers and a 8 series and above (+512 of ram) you can offload the hard work to the graphics card so low power cpu and graphics card does the work. Some people are using similar atom based systems as shown above and an Nvidia 8800gs with 512m onboard to play back 1080p.
Horses for courses, so your milage may vary. HTH
Regards
Chris