Building your own can not only be a cost efficient route, but can also be very rewarding. If you have the time, I'd say build your own. A word of caution however. Even though it isn't that hard at all to assemble a machine from parts, you do need to take your time and research the parts you intend to purchase. Learn all you can about compatibility, memory timings, clock and front bus speeds, power requirements, etc. This will not only help to eliminate compatibility issues, but also help you get the best performance for the money you plan to invest.

I always build my own, I can get various parts from various suppliers at great prices, then I can spend a Saturday assembling it, installing the OS and getting it running sweet and then spend all the money I've saved on software, beer and chocolates

Also, you can't get 8 Ghz out of any processor without liquid nitrogen or liquid helium as a coolant as well as a high-end motherboard.

If you build things yourself (or get a friend) you can usually get a better PC for the money that also has higher quality components. I don't see what's good about Alienware. The two problems with building it yourself or getting a friend, is if one part of the computer turns out to be defective, it can be difficult to pinpoint what part is. Also it's possible for people to kill hardware if they don't know what they're doing.

If you or your friend are not 100% confident in building it then get a computer store to build it using parts you choose. A local PC store near me offers this for $70, but I live in a different country...