Yes it will. In fact, many OEM and discount PC's that had full ATX towers came with Micro ATX boards.
Micro ATX is a standard ATX board with the last 3 PCI slots chopped off. This allows the company to shorten the board around 2.5 inches. When you mount a Micro ATX board in a standard case, you just have an empty spot that a longer board would have filled.
There is another, even smaller version of Micro ATX, called Flex ATX. It takes 1 more slot away! And there's one shorter than that called Mini ITX, that takes another slot away! Any of these will fit in larger cases.
<font color=blue>Watts mean squat if you don't have quality!</font color=blue>