Don't buy alienware... It's stupid, I wonder if they actually know how dumb they are?
I customized a computer, with the same specs as the one I am currently building, and my computer costed atleast $500 less.. Alienware is just the same hardware, over priced. Probably because it has a cool looking case, and the name is "AlienWare", they can boost the price up 400 bucks or so..
Also, all the sites you go to to customize your own computer have limited choices, and purposely make it that way so you don't spend too little money. It's just business, that's why building your own is the way to go.
If you are in to gaming, build your own, if your into word processing, get a laptop from Dell or HP. My dad just bought a new HP laptop like a month ago, and it is awesome. Very good. If you are into multimedia, you want something that will have a lot of storage, probably a Dell. Also, you probably want Windows Media Center.