I would reccomend you just buy the parts and build your own, you get better performance and save your self a few hundred $$$. I say if you have a friend then have them help you with it. Not really a complicated process if you have some clue as to what to do.
If you are going to buy one of the three I say probably Dell. Alienware charges an ass load just for you to own their cool box with average parts. They are nothing special. Gateway isn't what it once was as far as performance and service. Dell, you are still paying a decent chunk of change but they have awesome customer service, good pay plans and and some really nice warrantys. Just my two cents