Bear in mind XP doesn't support anything beyond 4GB. Dunno about Vista, but I've seen Linux running with 32GB+.
I don't think 4GB is suitable, really. Especially in a laptop, Alienware are known for just jacking their prices up and filling their machine with crap components. One of their offerings last year had a desktop P4 in it. Put out enough heat to cook on, and it wasn't even that good a machine.
In certain circumstances, more RAM can hinder performance. I'd say gaming was one of them, more RAM means more clutter and crap loaded into it, makes it harder and less efficient to find things. 2GB is enough, IMHO.