Exact usage requirements are a must to know when spec'ing a Media Centre. It will affect everything from the case you buy to the software you need to install (and purchase in some instances).
So, do you intend:
Backing up existing DVDs to hard drive Playing blu-ray films Use as a music centre Having more than one TV tuner (for recording / watching multiple channels) use of a satellite tuner at any point for Freesat Networking it to either an NAS or other PC
Also put some careful thought into what remote control you want to use.
I've got to say, I'm a huge fan of my Media Centre, despite having some issues that I'm still trying to resolve. Truly better and more versatile than any PVR, and not expensive either.