It sounds like your priorities are gaming and high-def movies. While there is absolutely nothing wrong with that, you need to ask yourself if you will really be using your notebook for those things, or if the idea just seems 'cool' to you. I don't mean that in an insulting way, I just mean that those goals may already be met by your ps3 and an HDTV. Also, the m15x isn't very portable, so you might want to consider a desktop for gaming and movies (much more power for your buck, but you can't watch a movie in bed or go to a LAN party as easily). The last thing to you need to consider is whether you use iLife and simply how much you enjoy OS X over Windows.
Basically, what are your needs? Gaming? You know why you want the m15x, but you should also think about why you want to be talked out of it. Think about your actual uses versus the attraction of having the most powerful notebook on the market. Also, I would go for the m17x if you're going all-out. It's not like the 15-incher is very portable, so you might as well buck up to the 17. Oh, and if you're really into power, they have some notebooks out with quad-core desktop chips... but that's nuts expensive.
-Ado