I have built several mATX systems in the last 6mo. or so and the best case I have used is the:
Fong Kai 611L ........ purchased from:
Directron
Price includes a 300W Enhance ATX ..... (it fits) ... psu ...........
Total depth measures 16.5" (including front plastic bezel) .......... 14.75" (not including bezel).
My second choice would be the Inwin V500 series ....... (close to the Fong Kai, not quite as good cooling options)
There are smaller mATX case options, but adaquate cooling is more critical and drive options and psu choices are limited.....