Two HD 7850's in crossfire and even overclocked will only consume a little more than 400 watts.
By 400 watts i mean not just the graphic cards but the whole system power consumption provided the CPU is not overclocked.
Look at the HD 7850 crossfire review to get a better ideahttp://www.tweaktown.com/reviews/4798/his_radeon_hd_7850_iceq_x_turbox_2gb_in_crossfire_video_card_review/index.html
The Cooler Master M2 series are pretty damn stable and at 75% to 80% load they can even achieve 90+
efficiency. Don't worry it will easily handle two HD 7850'sANSWER 2:
The CM 600w that you mentioned can handle your fx4600 but lets not forget that this cheap PSU with cheap components and very bad voltage stability under load. So it might cause you problems every now and then especially during electrical fluctuations or power outages.
I recommended you getting a cheap and good quality PSU like the Corsair CX430 V2 for this system.