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I built a mac by buying parts on Ebay. All you need is (usually) a mobo, cpu, and maybe power supply. The rest are regular parts.
You may also want to consider buying a basic machine from one of the major mfg's. Often, they start with a lowball price on the lowend unit and make it up by overcharging for bigger drives and more memory. You can usually buy a stripped down Dell and upgrade it yourself and have the same end result for $100 to $250 less than Dell would have charged.
If you are determined to do it yourself, I would recommend favoring functionality over speed. (e.g. buy a dvd burner instead of a really fast cpu).