Alienware is like buying a Mercedes Benz. It uses Chrysler car parts on a different body, beefed up, jacked up prices and calls it a Mercedes.
Same thing with Dell. Alienware uses simple Dell parts [XPS, Studio], beefed up processor, jacked up prices and calls it an Alienware.
The best thing anyone suggests is building your own computer by ordering your own parts and then finally building it. It could save you around 1-1.5k depending on your build. The problem is that a lot of people aren't knowledgeable how to build computers.
If you aren't down to build your own computer, then you need to start watching sales, which requires a lot of research. Probably the three most important components for a Desktop is going to be processor, video card, and amount of ram.
1) Compare processors.
Find different models and understand which processor gets more bang for your buck using simple mathematics to understand how much you're really paying for.
CPU-Benchmark
2) Understand if your video card is worth it and also know if its integrated or not. Problem with buying a pre-built desktop from these companies is that the video card is integrated into the motherboard meaning you can't, CAN'T, swap it out. Different video cards often require advanced chip sets in order to function properly. Comparing products will help you here.
3) 3gb Ram is not the same as 3gb ram. WTF DR YOU TALKIN' CRAZY!!!! Naw son, listen up. This article should sum it up for ya
ddr2 ram vs ddr3 ram
Basically the consensus is buy a desktop with a processor decent enough to run your games and programs, a video card that also meets those requirements, and finally having enough RAM so your computer can handle all its processes.
This decision should not be decided in a day. This could take weeks so I searched for the best possible deal on my Dell XPS. I found a deal one day thinking no other deal could beat it, but the next day I found a completely better deal. You gotta watch the deals and make sure you're getting a good deal from it.
ALSO
2012 is about to kick off with SOME HIGH TECH SHIT. First of all, CES 2012 started today. What does that mean? New processors, new computers, new operating system (Win 8), new TVs, new video cards are all going to be shown off at the event.
I would honestly hold off maybe just a little bit until this new technology rolls out. If your in need of a computer pronto then get to sales watchin'