Im in a bit of a pickle, here are my system specs;

CPU: FX 6300 with Corsair H80
Mobo: Asus Sabertooth 990FX
Case: HAF X (4 200mm fans, 3 120mm fans)
GPU: 2X R9 280X direct CU TOP
PSU: Corsair HX750


I have an old 7870 i have lying around, and im considering popping it in for a 400KHs boost. Im concerned that 750 watts isnt enough for the power draw of my system due to all the other various componenets that i have. So far i calculated with these 3 cards i need around 755 watts.

I was wondering, what can i Under-clock to take some pressure off my PSU for mining. I dont really game too much, i just use my system for mining/school work. So far im planning to undervolt/declock my cpu to 3.0ghz, and lowering core frequencies on the GPU's/increasing memory frequencies.

Any other possible ways to shave off power you guys have in mind?

-Thanks!