diehardware basically sums it up in a nutshell. Personally, I'm gradually moving all my boxes to M-ATX or SFF sized boards. OC is still possible if you select carefully. I sell or give away most of my hardware after a year or two and let's face it, it is easier to sell or get someone to take a slim or compact case than a big beige box. I also tinker with making my own cases out of plexiglass, wood, cardboard boxes, drawers, etc. and a M-ATX works well. I don't need the extra slots of an ATX because most of what I need is built into the m/b and if I want to expand, 3 slots are plenty.