In my last post I gave you guidelines for building your own custom computer; now I'm going to give some advice on purchasing parts for your computer.
The first and most important thing to do is set a budget; unless you're a multimillionaire, you'll want to do this so you don't go broke buying parts. After you've decided what you want out of your computer, research the right parts for it. Then you'll have to research places to buy; the cheaper the better. Many times, "generic" brands are just as good as name brands, but below you'll find a list of reputable companies to buy certain parts from.
Case: NZXT, CoolerMaster
PSU: SeaSonic, Corsair, PC Power & Cooling
RAM: Corsair, OCZ, Patriot, G.Skill (usually the cheapest)
GPU: nVidia- eVGA (best there is), XFX (great warranty), BFG; ATI- Sapphire
Disk Drive: Western Digital, SeaGate, Maxtor, Samsung (some people will argue Maxtor, but I love them)
After you've determed everything you need, from the parts to the manufacturers, you'll need to decide where to buy from. Now, I'm not going to give other websites to buy from because I don't want NewEgg to hate me. I'm not saying NewEgg isn't amazing, because it is, but sometimes other sites have better deals. Search around. If you have questions about the reputability of a site, send me a PM.
**Cautionary note: Be extremely careful when purchasing from eBay. Do extensive research on the seller, paying particular attention to feedback. I would advise against buying from someone with zero feedback unless you're really daring. Remember: the higher the positive feedback, the better the seller, and the more likely it is you'll get what you're paying for. Avoid bidding on or purchasing overclocked items that are not new in a sealed box. Lastly, read the description carefully; if there isn't much of one, I'd avoid buying it. The longer and more in-depth the description, the more reliable the seller.