I've been looking further into Linux media centre possibilities. I have to say that LinuxMCE which I mentioned above probably isn't ready for non-geeks. The Google video was impressive, but it glossed over the many rather difficult configuration requirements. And it's probably over the top - it's not just a media centre, it's a complete home control system for everything including the kitchen sink.
For a straightforward media centre, MythDora looks like the bee's knees to me. (The daft name derives from MythTV, the well-known but difficult software for media centres, and Fedora, the Linux distro it's based on.) MythDora automates the difficult bits, and has excellent documentation for the rest. I'm installing it on a spare PC at the moment, and I'll let you know how it goes.