The XPS would be a good choice. Try the XPS M1710 though out of the three XPS notebooks. I don't have any experience with the Dell XPS systems but they look cool and should run great. Also, Dell has just about the best customer support which could come in handy at times. Furthermore, The intel core duo processors are amazing. I recently bought a Macbook Pro and installed Bootcamp and Windows XP Pro and games ran great.
As far as Alienware is concerned, I've only known two people with Alienware laptops and although they look cool and have decent specs, both people had to ship them back to Alienware with fried hard drives and other misc. problems.