Computer parts are a volitile commodity, unfortunately. You really have to decide what your budget is, what "level" of computer you want to build (all out gamer, midrange gamer, web surfer only, etc) before you go shopping. THEN you either buy your parts piece by piece over time, or you bite the bullet, buy all the parts from one source "on sale" at the same time and don't check back in the coming weeks to see if you could've saved $10-30.
That said, the best piece of advice I can give you is this: In any computer the most important piece of hardware is the power supply. With a cheap power supply, even $4,000 worth of parts will not work worth a darn. No $35 case comes with a good power supply. Expect to spend $50-$60 MINIMUM on a good power supply ALONE. Case is extra. Remember: A good PS and case will last you through many "new computers." Good power supplies are made by (not in rank order): Antec, Seasonic, Corsair, PC Power & Cooling to name a few good manuf's.