One thing about Alienware that you guys keep forgetting is that you can turn around and sell the system on ebay with no problem when your ready to upgrade. You might be able to get a good bit of your money back. Not all of it by any means. People will pay for it because of the name on the box. Your probably not going to get one single bid on your DIY box because people who don't know computers are going to be looking for name brand stuff.
Not that your pc isn't as good as Alienware, but most don't know that. Part of my job description is building 2 or 3 pcs a week, and frankly it get's old. Building a pc get's mindless after a while. I may very well have some one else build my next one for me.