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The key is to SHOP around for quality parts, and be very meticulous about applying for rebates. Use only quality e-tailers that have a good reputation, I suggest: http://www.newegg.com, http://www.tigerdirect.com, http://www.buy.com, http://www.provantage.com, etc. (I do not work for any of these companies, your mileage may vary.) Also be sure to check your local shops, sometimes if I catch an item ON SALE or with a REBATE, it will beat or match online pricing, and you won't have to pay shipping. I have gotten some good deals through the years at CompUSA, MicroCenter, Best Buy, Circuit City, etc.
Okay, so now that you know how to shop.... how do you know what are "quality parts" and what you should be looking for? DO YOUR RESEARCH! Before buying anying, research your items on the hard core tech sites such as: http://hardocp.com, http://anandtech.com, http://tomshardware.com, http://extremetech.com, etc. Search the newsgroups, http://groups.google,com, and make sure people aren't having massive problems or driver trouble with the component you are considering buying. (Especially if you use weird OSes like Eugenia! Make sure there is driver support for the oddball-OS you are probably running if you read OSNews.)
That's all for now, my fingers are tired. Some other hardware geek can address putting it all together.