This will quickly become a multi-reply thread of "Don't buy alienware", so- allow me to begin: "Don't buy alienware!"
Are you dead-set on buying a pre-built PC, or are you entertaining the option of building your own PC?
What if I told you that you could build a better PC than that Alienware x51- for the same price? That alienware doesn't have a graphics card (which kinda defeats the purpose of alienware doesn't it?), so it won't be a steady performer on newer games- if you are looking to play games... which I believe is still alienware's target market?
Also, need your definition of small- if size is a big factor, I want you to have a case of around this size if you built your own...