My only issue with XPS/Alienware is that they are VERY overpriced. If you're looking to play games (since this is why most people buy XPS/Alienware machines), a custom-built desktop is really the way to go, as laptops are always going to be a little behind in the hardware department. Custom-built machines always end up being a better value for your money as well - you get to research and select your own brands and components, and the prices aren't jacked up. Building it yourself is easier than you think. You can do it with a simple online guide, and a general rule of thumb is: If it fits there, it goes there. ^_^
As far as hardware goes, it depends on what you want to be doing. Most new PCs come loaded with Vista (which sucks), and Vista is rather power-hungry. If you plan on watching a lot of movies, doing some intense photoshopping, or playing video games, I'd recommend 3-4 GB of RAM in a Vista machine, as Vista will eat up more RAM than an XP machine would. Also, for gaming do keep an eye on your video card. Look over some quick customer reviews for the newer versions of NVidia and ATI cards to make sure you're getting the one you need. The processor speed is also rather important, but most machines nowadays are a decent speed dual core or quad core, so again it all depends on what you want to do on your machine. Some quick research on hardware and how it works can help give you a better idea of what you need and what you should be paying.
I'm all about research, as I had a Dell XPS two years ago that completely fried on me 6 months after the purchase. After some digging, I found out that Dell at that time primarily used generic parts - whatever they could get that was cheapest for higher profit margins. I have been told that Dell very recently stopped doing this and is using more reliable hardware now, so maybe they are worth looking into if you're really against a custom desktop. However, ever since my hard-earned $3000 machine went up in smoke, I have been all for the custom PCs. I was an ignorant shopper back then, and I have since put a great deal of time into being a smarter computer geek.
If you must get a laptop though, I suppose there really isn't a totally wrong route to take, as it's my opinion that no laptop operates all that well anyway. My old college laptop is an HP, and it runs well enough for what I need it to do: Word processing and internet browsing. I guess I'm just spoiled with the desktops