At first glance, it might seem odd to suggest upgrading a system by replacing the case. When you think about it, though, there are some good reasons to replace your computer case, including:
Older and inexpensive cases were designed with little concern for noise level. Many modern cases are engineered to reduce noise levels, with features such as sound-deadening panels, unobstructed air intake vents, large fan grills, and rubber drive mounting blocks to isolate vibration and noise. By itself, a good case can't make a loud system quiet, but it can help reduce the din. When combined with other "Quiet PC" features such as a quiet power supply, quiet CPU cooler, quiet case fans, and so on, a well-designed case can help you build a nearly silent system.
Modern processors, video cards, and other components run hot. Older cases, designed in a time of 30W processors and 10W video cards, don't cope well with the heat load produced by modern 130W processors and 100W video cards. Modern cases are engineered with unobstructed air flow, shrouds, and other features designed to maximize cooling efficiency.
Standard computer cases are, not to put too fine a point upon it, ugly. Nowadays, good case companies have engineers to worry about noise levels and cooling efficiency, but they also employ industrial design experts to make sure that their products are also visually attractive. The best modern cases are attractive enough not just to have on display in your office, but in your living room.
And, of course, most new cases come with a new, higher-capacity power supply, which is no bad thing. If you're not prepared to replace your case, though, there are several improvements you can make to your existing case.
Can't find what you need? Try this upgrade and repair index.