Don't get an Alienware. They're extremely overrated. If you're going to dish out that kind of money for a computer you'd be much better buying the parts individually and assembling it yourself; much cheaper and you don't get stuck with too much crap you don't need and a fugly case.
Now, as far as Vista is concerned, I'm waiting. There's nothing I particularly want in it aside from DirectX 10 and games aren't even using DX10 yet. Unless you're buying a new computer, I suggest you just keep whatever OS you're currently running until Microsoft releases some service packs to correct any stability issues and bugs that, I assure you, are present in Vista.