like what they said, that PSU was suspicious.
there's only ~250W(unstable) power that you can get from those "generic 500W PSUs", so think of it how much that 450w same brand replacement can give you. and so did it's +12V rail amperage.
those 4000 series cards are power hogs compared to the new ones that we have to date. you need a clean/continuous 300W of system power at-least to run that card effortless.
get a decent PSU