Need help picking a power supply

Hello, I'm looking to upgrade my current psu so that I can in turn upgrade my graphics card.

My computer is an HP p6975a
here's a link to all it's specs http://h10025.www1.hp.com/ewfrf/wc/document?docname=c02...

The current power supply is-
Internal 300W (100V-240V)

Form factor: Internal ATX
Total wattage: 300W
Nominal input voltage range:
100-127V/8A (50-60Hz)
200-240V/4A (50-60Hz)
Dimensions: 150mm x 140mm x 86mm (5.9 x 5.5 x 3.4 inches)

Unfortunately I don't have much knowledge on this subject so if someone could suggest a psu that would be suitable for my computer, preferably around 500watts (to my understanding that would be suitable for most mid range graphic cards?)
Any help would be great!