Take a look at this power usage calculator. Just one of many that are out there. There is a sticky around here somewhere with some more. Plug in your information and see what it comes out.
Make sure that you not only check the power required overall, but what you're going to need on each 'rail' (12v, 5v & 3.3v lines). If you take a look at alot of the 'my m2-atx is dead' threads, you'll find that alot of people have only considered the total power consumption, not the requirment on each rail and fried their PSU.
The m2-atx specs outline the maximum output on each of the rails. Don't exceed those and you should be right