But the combo only has a 750w power supply which concerns me with the crossfire
750w is plenty
http://www.anandtech.com/bench/Product/510?vs=508
a single 7970 uses 357w for the entire system when under load. So if you attribute even 300w of that, which it would be way less than that, you're still only pulling 650w, which leaves a good bit of headroom. We recommend about 100w at least headroom i think normally, and this achieves that, normally we give a little more actually going by this, never paid too much attention, just picked up on the normal numbers and said those, but now i know why thanks to this
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For overclocking, youtube is a good thing but asking in the cpus and overclocking area is better. you can get a better explanation there, but to sum it up, you don't need to overclock the gpu at all but you probably can, as for the cpu, you must up the multiplier by .5, adding .05ghz to the clocks, then boot and run prime95 or another cpu heavy bench to stress the processor, while monitoring the temps to make sure they don't hit ~60 normally so you know you're safe, then reboot raising the multiplier again and checking again, then rinse and repeat until desired speeds with good temps are reached