Almost all motherboards nowadays are built to the ATX standard. The bad part of this is that you are unlikely to be able to use your old case and power supply. For another discussion of motherboards see this . The good part of this is that cases nowadays are much better than before. Avoid like the plague cases with a wrap around, three sided lids, or that require screws for structural support.
Life is too short to spend it struggling with sheet metal.
For a little more money you can get one with a slide out motherboard tray, extra fan spots, swivel side doors, extra fan points, extra hard drive locations, and tool less opening. Its amazing what they can do with sheet metal nowadays. For some reviews of cases go here , or here , and for recommended features here . The second great under appreciated component is the power supply . Usually they come with the case, and no more thought goes into it.