If you haven't bought it yet... I suggest you look at other PC's first.
There are some awesome PC's you can build yourself, with getting double the performance for the same amount of money. Don't jump into gaming with an Alienware because they seem cool and fast, because you can build your own, very easy, and get better performance for less money.
Also, it's very hard to upgrade cooling in the X51 because it's such a small case, it doesn't fit any aftermarket coolers..