Without question, do not use anything other than nVidia for your graphics hardware... ideally from the outset, and get a mobo that already has nVidia hardware.... its a no brainer
simon_b - that is a simplistic assessment. There are MANY features of LMCE's media-only functionality that are difficult/impossible to replicate in other media centres, without even looking at the other stuff LMCE provides. Such as, an interconnected/distributed media platform using MDs; virtualised remote control interface (on-screen, remote, etc) of all media devices around the house from any other media device, PDAs, mobiles, web pads, web browsers, PCs ... even custom devices that can trigger specific purpose DCE commands. And this is all _before_ you consider the interaction between the media environment and other features such as home automation and telephony (starting media playback triggering house settings like lighting levels, telephone calls pausing media, or automatically being redirected to voice mail to prevent interuptions, etc)
I have toyed with many of the other facilities that LMCE offers, but right at the moment I am only using the media facilities and I still would not consider moving to another "media player" platform... its not as simple as media-only=another system as anyone who has used LMCE for a while will atest. Sure, if your primary concern is a slick, eye-candy GUI and are not bothered about anything else then XMBC becomes a good alternative, or even Apple TV if you are in the camp of UI is everything, functionality is nothing!