Question regarding GPU and shitty Power Supply

Hey guys, I just bought a new GPU R9 280x and I connected it to a Sentey 450W Power Supply (very shitty). I had been using it the last couple of days underclocked without any problems. I also had connected both power pci express conectors (don't know if both are required, would like to know aswell).
Problem is my PC just shutdown and won't turned on for a brief period of time (5 mins). I'm now using the onboard card.

I have a shitty CPU Athlon II X2 250 so it doesnt use much power consumption. I'm only using this PSU until I get a new one next month, can it hold on that much? Can something go wrong if I reconnect the GPU?

Get a minimum 500w 80+ bronze certified - single rail PSU.

I like seasonic and XFX because XFX uses seasonic's parts   lol

I don't care if I sound like a 'fanboy' ... they speak for themselves

And you can't beat the lifetime warranty I got on my Ultra X4.

Ideally you want to be at about 80% usage of available capacity. 

Too much is better than not enough.

Under powering your GPU may have caused damage to it.