You should look into MythTV. It's Linux based, but probably quite a bit better than Windows Media Center... Which from what I've read, tends to be big on promises, and not quite so big on delivery.
If you're looking to use this system as a desktop as well, don't bother with MCE. There's nothing about it you couldn't do with XP Pro or Home.
Given Microsoft's usual development time to get things right, it'll be another 2-3 years before MCE lives up to the claims Microsoft is making for it now.
I am considering building my first computer and would like to use Windows Media Center Edition. I've read it can be a little finiky with some hardware compatibility (which I think I could handle), but is it hard for a first-timer to put together a reliable WMCE computer from a software standpoint? I have some experience but would not consider myself advanced by any means.