Look for something in the intel core 2 duo line, they're top dog now. And honestly, i don't recommend Alienware. They charge a buttload of money for something that could be bought for a lot less without the brandname bullshit.
And also, if your main goal for a computer is gaming, unless you do a lot of moving around, travelling, etc., i don't see the reason to shell out £2500 for a laptop when you could buy a top of the line desktop PC with that kind of cash.
All work and no play makes me..., well, you know the rest.