AakeKaake :
So apparently i.e. SeaSonic is pretty good manufacturer?
For development of this guide: What should high-end pc users consider PSU-wise when processor OC'ing and Crossfire/SLI comes in picture?
Whats good in wattage for SLI GTX 780 Ti or CF R9 290X?
Nvidia has a pretty good reference guide for this, I'm not sure about AMD.
You should just google "PSU calculator". There's a tool created a while ago that will let you input the devices and/or their wattage for your whole system and then figure it for you. You usually want to overshoot that amount by 20-30% if you intend to add on down the line.
For my system (in my sig), the minimum would be around 700W functional wattage (under load and real output) so a 750 80+ Gold would be required or an 800 80+ Silver. I opted for an 850 because the cost difference was minimal and I wanted some headroom.
The rating is a big factor. If your PSU is only 85% efficient, for example, then your 700W might do 600W under load if you're lucky; if it pulls more you're likely to blow a cap or the transformer down the line.
Do some reading on reviews to see what the real world efficiency is of the PSU first.
Seasonic is always a reliable bet. They often out-perform their 80+ rating.