How or where do you learn what parts make a quality computer?
A couple of great resources are a magazine called Maximum PC, and a website called anandtech.com .
MaxPC gives reviews to the newest and best computer parts with an eye towards high end performance and gaming, as opposed to office type number crunching computers. They also have an issue each year where they build the "ultimate" machine for that year, and the "budget performer" for that year. The ultimate is ridiculous - like $20,000. But the budget performer is a high quality machine that is just a few steps back from the very latest breaking technology - but is still far better than what you could buy off the shelf at your Best Buy.
Anandtech.com is much more technical and is definitely for the geek-at-heart, but EVERYTHING is reviewed there in gory detail.
To build a completely basic computer you need a : power supply, motherboard, processor, hard drive, memory, video card, and a CD-ROM drive for loading an OS. To build a smokin' fast gaming computer you need a nicer video card, more memory, faster processor, and potentially a nicer motherboard with some more advanced features, though I'd place that last on my list. Your BIG improvement is going to be seen in video card and memory, in my opinion.
A few things that help. Fast internet connection, good motherboard, a good cdr/rw/dvdr/rw and it's better not to buy your motherboard from a store (ex. Bestbuy).
Another way to shop for components is to go to a benchmark site. There, you can see how all the main components compare in performance, and thus you can balance price with performance so you can build an affordable computer with the performance that you want.