Cheap Computers?

Okay, so i know some people who claim they can make a computer for 400$ How come then alienware etc cost so much?

What parts would I need to make my own computer?

You can't in good conscience recommend an intel to an amd, sure, intel's are quad and amd's are dual, but amd's (i dare you to disprove this with 2 comprable chips) run cooler, faster, longer, easier and more stable in overclocking, AND cost less than intel.  do say it simply, intel chokes on limp dick, take it from some one who has had quite a bit of experience with both chips (i primarily use intels, but some customers are REALLY thick, so i give it up and let them live with an inferior product).  There is a reason that in all of my rigs I use amd, but dont take my word for it, check the specs, and swing by andandtech.com, they do a great job benchmarking products.