How to buy a computer case
For most people building a new computer, choosing the case is either the least important decision or one of the absolute most important decisions. The case is, after all, one of the most visible parts of the system, but it's also the least technologically complex. At the end of the day, most modern cases are still just sheet metal and plastic crafted into the approximate shape of a rectangular prism, with some bits inside.
However you prioritize it, your computer needs a case. Even if that case is an anti-case. So let's dive in.
The first thing you need to do is lay down some basic constraints. This will make shopping much simpler, because you have a general feel for what you are looking for. The first constraint is normally cost. It is perfectly possible to spend anywhere from $50 to $500 on a case without going to any great effort. This is purely a function of what your budget will accomodate; more money will in fact buy a nicer case, but there are strong diminishing returns after about the $150 mark. Most people spend around $50 to $100 on a case.
The second, related, decision to make here is on the power supply. Many cases now include a power supply, and if you are looking for the absolute-cheapest way to build a system, this is the way to get there. However, the supplies included with cheap cases are usually utter garbage. Shopping for cases that include a power supply is fine, but you will need to increase your budget (buying a good power supply on its own typically costs about $50 to $100) and shop carefully.
If you really just want something that will keep all your parts in place with no fuss, budget $50 for an inexpensive Raidmax or Foxconn case, buy a separate power supply, and skip on down to the bottom of this page.
Those two choices should be enough to pare your choices down from the 800 or so cases that Newegg currently stocks to about 200. This is progress. But to get down to a more manageable number of options, we need to introduce some more factors.
For better or worse, this is the primary motivator for many people. This is purely a matter of your own personal taste; some people want bling, some people want utilitarian design. There is no right or wrong here, although if you live with other people now is the time to consult them on their own aesthetic preferences.
The basic job of a case is to physically protect the computer and to help it stay cool. Cooling, then, is a major job. A lot of smaller cases have real cooling problems, often because their only air intake is a single 80 mm fan at the front of the case, obstructed by some sort of vent. This is generally down for looks (fans are not, as a rule, pretty) and costs (small fans are cheap).
For most uses, it is easy to prescribe an adequate cooling setup: a 120 mm fan at the front of the case (without too much blocking the passage of air), and another 120 mm fan at the back of the case. There are cases where this setup won't work (either won't provide enough air flow, or the fans won't physically fit), but those are uncommon. The preference for 120 mm fans over smaller fans is simple; larger fans can move more air while turning slower. This means they make less noise, and the noise that they do make is lower-frequency (and therefore less annoying).
Some cases will have fans on the side or top of the case (sometimes very large fans), and these can also work well enough. The main factor is to make sure that you have a feel for how air will flow through the case (and especially how it will move around the processor, video card, and memory - the hottest components) given all the fans involved. In standard ATX cases, it is typical to have air move from the bottom-front of the case to the upper-back, but other arrangements have been tried and can work.
Do you want a big case or a little case? Big cases can hold more stuff (more drives, bigger video cards) and are easier to cool; small cases are more portable and often very challenging to cool. Very small cases may not permit the use of many standard components. Buying something substantially bigger or smaller than the norm will cost more than a generic, "normal" ATX case.
If all has gone to plan, by now you should have a list of choices short enough that you could write it out without collapsing from exhaustion. There are a thousand minor factors that people use to whittle this list down to a final choice, but we will cover some of the more important ones.
Love them or hate them, doors grace the front of many cases. They are intended to conceal the optical drives (which often do not match the case) and pretty things up, and generally succede at that. Badly implemented doors may block air intake to the front fan(s) or have hinges that break easily. The door also has to be opened for the optical drives to be used. Some cases now include drive-bay covers that cover only the drives themselves, so that the drives can be opened with no extra effort.
Windows have become popular over the last few years as a way to show off a computer's guts (and in response, hardware manufacturers have made all sorts of fancier-looking products to cater to that market). Many cases now have a window on the side panel. On cases that don't, it is of course possible (but takes a bit of doing) to manually cut out some of the metal and add a window yourself. Windows are often associated with lights or lighted fans, which some cases include but which are usually purchased separately.
Most cases are made either of steel or aluminum. Steel is generally cheaper, but heavier. Different thicknesses of steel may be used. Aluminum cases are more prone to problems with rattling, because of the thin, light sheets of aluminum normally used for computer cases. They are also more easily dented. A small number of cases are made of acrylic, which then leaves the entire contents of the case visible. Acrylic cases can have issues with scratch marks (which are fairly visible and get worse over time) and with warping (they may not be adequately built to support heavy components).
More expensive cases are, as a rule, built to higher quality standards. The parts fit together better, metal edges are sanded down (or folded over, even better) to prevent cuts, and they are simply nicer to work with. You are also more likely to get niceties like extra screws. Not long ago, more expensive cases often touted their "tool-less construction" (that is, users could install a system using few or no tools) relying on thumbscrews to hold case panels on and plastic latches (or other systems) to hold drives in place. These have become less powerful selling points to most users, but are still offered by some cases.
Some manufacturers have stronger reputations for construction quality than others. Lian-Li, Silverstone, and Cooler Master are above average; Linkworld, Xion, and Raidmax are below average.
So you've narrowed it down to your top three, say, and you're ready to start spending some money. Good! The next step is, unfortunately, a little bit more shopping.
Because of their size and weight, cases are one of the few computer components that are still sometimes a better deal when purchased at a brick-and-mortar store. So you may save yourself some money by comparing prices at local brick-and-mortar stores to sites like Newegg and Amazon (affiliate link), being sure to factor in the cost of shipping from the Web vendors.
One last tip: some cases are sold, with minor or no modifications, under multiple brand names. This is especially common with Raidmax and Ultra brand cases, and less common with higher-end stuff. By keeping an eye out for look-alikes, you may be able to snag a deal.