How do you know your system is shutting down due to a low power supply??? Also what tools /software are you using to guage temps? Is the CPU or GPU overheating or both? Is there an overclock on either the CPU or the GPU or you running both at stock speeds?
Also what is the make and model of your current PSU?
Sorry for all the questions but I need more information in order to help you
I have just ran WHAT I know about your system ( assuming you have 8gb RAM a HDD and an Optical drive ) through a PSU calculator and maximum power draw for your system should be approx 315 Watts. The obvious PSU choice would thus be any decent 500w PSU that is at least 80+ Bronze rated. That GPU draws power form your PCI-e slot rather than from your PSU directly as far as I know? 500w will give you adequate head room for adding on additional components/peripherals and overclocking should you feel the need.
IF your PSU is at fault ( it may well not be) then this little one here should be fine for your needs.
http://www.amazon.co.uk/dp/B00H33SFJU/?tag=xov-uk-21