It does not work how you are thinking, very often when you buy a cheap unbranded power supply Jimmy.
It often does not output no where near the maximum amount of wattage it quotes it can provide.
And even if it can due to cheap components used inside of the PSU such as sub standard capacitors of a low grade.
To keep the cost of the Psu cheap to buy the result is when the Psu is put under high subjected power demand or load by the rest of your system components such as the ATI 390x card you have, especially when you run a game.
The power delivery of the Psu can be unstable or fluctuate wildly.
And often the cause why a system will crash with such a powerful graphics card being used, especially if it happens when playing any game on your system it causes a system freeze or complete lockup of your system.
The other danger point is this, a cheap unbranded power supply to save costs will also have limited power protection circuits build into it, for short circuit protection, over current protection, or surge protection from the mains supply, wall socket it is connected too.
Consider this for the cost of a graphics card a 390X and the rest of the system and what the total cost.
If the cheap power supply developed a fault.
Where it overloaded and took the rest of the parts of the system like an expensive 390X card.
How much would it cost to buy the whole setup again if the case ?
Vs paying a bit extra for a well branded power supply, at a good power rating with lots of circuit protection features designed to prevent damage to other components of your system ?
Is it worth that £100 or dollars you may have to spend, other than another £700 to £800 to replace all of the parts a cheap PSU took with it ?
I think you have your answer don`t you. ?
Here look at this PSU bellow, click on the link.
Then see what protection and ratings the cheap power supply unit you are currently running with the system has, if your in doubt.
https://www.overclockers.co.uk/corsair-sf-series-600w-80-plus-gold-modular-sfx-power-supply-cp-9020105-uk-ca-218-cs.html