My general advice is pick up a solid unit from a reputable brand. Corsair, Sea Sonic, EVGA, and a few others all have good units. For wattage I would say 600w is a good place to look for a lot of builds. Higher efficiency power supplies are typically more expensive but they do typically have better components. I personally like EVGAs gold rated power supplies. I have an EVGA G5 or G+ 850 and I've pushed well into the 700-800w territory on it before and it took it like a champ. I had a 5700xt red devil, R9 Fury Nano, and a R9 Fury X in a system with a 2700. All cards were overclocked and some back of the napkin math tells me I was pulling "a bit" of wattage. I do like corsairs digital power supplies because they allow you to make the system turn off if temperature gets out of where you want them to be.
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