You can get decent gaming laptops these days, but for the best gaming experience, and much cheaper a desktop is the best option.
Alienware are good, but you pay alot for the name, you can get just as well built and powerful systems for much cheaper.
Also, Macs and Mac books would be good for gaming, if most games were made for them which they are not. As performance wise they are no slower than Windows based PC's. In fact the top end Mac pros come with two Intel Core I7 processors giving a total of 12 processing cores with hyper threading which means they can handle a massive 24 processing threads at once. Windows PC's can't do that.
That would be gaming bliss, if games where made for Mac's.