Graphic card help? gtx560 or 7850

hi guys i need help choosing a video card which is either the gtx 560 or the sapphire 7850 1gb non overclock.
the 7850 that im looking at, uses less power than a 560, however the PSU requirement is 500watts which my PSU is exactly 500 watts.
so is it worth the risk to get a graphic card power requirement exactly the same as my PSU watts, which is brand new btw?
im thinking since i get about 38 amps on the +12v rail which i checked myself and i do get about that, i should be fine with that PSU, since i only have 2 optical drives, mobo with 3 fans, 4gb of ram, and a gt240 which pulls 69 watt thats being powered-SOON TO CHANGE to either the 7850 or 560.

Now since im getting a new graphic card in general thats for gaming does it mean my cpu is going to be doing more work WITH THESE NEW CARDS THAN WITH MY GT240(OR WILL THE WORKLOAD BE THE SAME) hence its going to get even hotter now, IF its going to be doing more work with these cards?
here is the 7850 that im looking at

http://www.microcenter.com/product/400655/11200-16-20G_...

what is ur opinion on 7850, drivers wise and reliability?pros/cons
i read alot of new egg reviews and they say the card is quite cool but those reviews were the over clocked version, and this isnt the overclocked version so it should run even cooler because(over clocked and normal version of this card seem to have the same cooling system) it only uses 130 watts.. ill either get this 7850 or the gtx 560. dont worry about y im basing my vid card on temperature. so thanks again
link to psu
http://www.microcenter.com/product/399981/Builder_Serie...