A lot of you will probably have differing opinions on what is important for you in a PC case. Heck some might even want to build their own custom case from scratch, which mind you is pretty damn awesome if done right. Things I find important in any decent PC case is build quality of the case, you don't want it to be flimsy or full of rattles when the pc starts up. Space for all your hard drives and other goodies is also important, especially making sure it's big enough to fit whatever graphics card you want. Prices on PC cases range from very cheap to stupidly expensive, I've seen some cases go for above R6000.
Some possible options to look at is the Gigabyte IF 133 Gaming Chassis a no frill sturdy case that is cheap in price but not quality. The Coolermaster N300 slightly more expensive than the Gigabyte but has front USB 3 ports. The Coolermaster Force 500 ATX Chassis with CM Elite 500W ATX 12V V2.0 PSU which comes with a 500W PSU at a decent price. The Raidmax Smilodon which has been a firm favorite for those looking for a bit less boring case and the Zalman Plus ATX Mid PC Tower Case - Z11 which features some of Zalman's renowned hair cooling technology.
Most PC cases come without Power Supplies and usually those on the cheaper end of the scale, come with really weak 350w PSU's. The power supply is a very important part of your Rig, as it's the heart of your machine. Making sure all your hardware has enough juice to perform at their best. You get really cheap and really expensive PSU's out there. Just never buy one from a mostly unknown or unheard of brand. For the Most gaming PC's these days a proper 500W and upwards PSU would be all you need, depending on how many GPU's you want to run.
Some possible PSU's to consider are the Coolermaster Thunder 500W, Antec NeoECO 520C, Thermaltake LitePower 650Watt and the Corsair Builder Series CSM650.
One thing to keep in mind regarding PC cases and PSU's are, they are usually components that you can keep for a very long time IF you have decent ones, so don't be afraid to spend a bit more on them. In part 2 I said I would probably look at storage options and drives in this part but I decided I'll leave that for part 4!