When I built my first MCE there was not really MCE systems that "fit-the-bill" as it were, I started with a basic PC, that evolved into a HTPC, and later that involved into "big bertha" my 7.5TB dual XENON Media Center monster.
Now although the above experiences were fun for me (although at some points I dont think fun describes the experience too well) if I were to do it today I can't say I would take that approach as there are a numberof high-end media center systems out there that can deliver what I would want out of the box, these include:
More over these vendors are working to "push the limits" on what Windows based Media Centers can do today, and as a beleiver of voting with my pocket book I think its the right thing to work with folks like these so they keep up the good work (e.g. they must make money to continue to invest in these areas).
In any event if your thinking your going to go do a high-end MCE seriously take a look at these guys...