In 1998, Ask Ars was an early feature of the newly launched Ars Technica. Now, as then, it's all about your questions and our community's answers. Each week, we'll dig into our question bag, provide our own take, then tap the wisdom of our readers. To submit your own question, see our helpful tips page.

Are you ready to build your own system? After looking at hardware and software, in Part III of our Ask Ars DIY series, we look at cases. What are the factors you need to consider when deciding on a case? There is a lot more than good looks to consider—things like airflow, cooling, space, and even the drive mounts. Be sure to read this before you buy a case.

Computer parts don't need to live in a plain, boring box that keeps your parts off the ground and dust-free. A good computer case—or chassis—has to keep its contents cool, but not be too noisy. It also has to be the right size, while fitting your budget.

These are tall orders: with enough money, anyone can have a very nice chassis. For the God Box, it's easy to justify spending a couple hundred bucks on a nice case with the right fans and layout. For the Budget Box, that same case would be half the cost of the entire box. So you've got to be flexible.

Case considerations can be boiled down to size (external and internal), cooling, noise, and cost. Looks matter, too, but who wants a sharp-looking, cramped, noisy case?

Case details

The basics

Space (external): Where is the system going to live? The space available for a case is obviously a huge factor. A large case in a tight space makes for venting issues. No matter how good the airflow is inside the case, if cool air can't get in and hot air can't get out, it's not going to work. If you can get six inches of space on each side of the case, or at least on the sides with vents, that will help, and if you can provide a lot more space on the front and back, where the main air intakes and heat sources often are, that's even better.

Space (internal): Making sure all your components fit seems like a no-brainer, but the question of space works in two different ways. A case can be "too small" in obvious ways: if your dream chassis only fits mini-ITX motherboards and your dream motherboard is standard ATX, it's not going to work. Love a particular micro-ATX case but want to fit three high-end double-slot video cards? That won't work either. Long video cards? Could be an issue too.

The "too large" issue may seem less obvious, but a case that's a few sizes too large means that you have a very large volume of air inside the case. Heat from your components rises, and if the fans in that oversized chassis aren't anywhere near your hot components, things may become uncomfortably warm. You may also find yourself adding extra fans in order keep air moving in the right places, adding to the noise, cost, and wiring mess inside.

Power supply (fitment): Make sure your chassis fits the model of PSU (power supply unit) you intend to use. Some smaller chassis can fit larger PSUs, but not all do. Most ATX chassis can fit a standard ATX PSU, but some power supplies tend to be longer than standard. Enthusiast chassis tend to accommodate this more readily, but always double-check.

Cooling and noise: These two issues are so interrelated we're going to discuss them together. What's state-of-the-art in chassis has evolved considerably, but some general trends exist. Fans have gone from small, cheap, high-RPM, noisy units to larger, slower-spinning units that move air around with less noise.

80mm axial case fans used to dominate; today, 120mm fans are everywhere. 140mm and larger fans are occasionally seen too—and who knows what the future holds? A slow-spinning 120mm fan may not move as much air as a screaming 80mm fan, but it will almost certainly make less noise. And it's not just how much air is being moved around, but whether it's enough air being moved to the right places. Your hard drive may become unhappy if it soaks in its own heat, but just a little bit of airflow is usually enough to keep things cool. Placing your hard drive at the edge of the airflow of a nice, quiet, slow-spinning 120mm intake fan that also happens to be blowing cool air at a hot video card might provide enough airflow for your components.

Sometimes you'll need more cooling. That's when you'll need to look at high-RPM fans (which will be noisier than slower fans, all other things being equal), or maybe just more fans. Consider cooling with care.

Cost: The generic case made out of the cheapest stamped steel might fit everything, but will it hold up? Spending the money on a quality chassis that is rigid enough to control vibrations and not cut your fingers due to sharp edges is always nice. Proper fan mounts with fan grills that let air in and keep fingers out without obstructing too much air and creating too much noise are nice too. Filtered air intakes? Those cost more too, but keeping dust out is always good. It's a balance of cost, design, and features. Find the right balance and things can be worked out—be realistic!