I searched for your machine on HP's website, it's listed under specifications. It is also listed on your power supply. To give you a better answer, we need the numbers of the rails off of your power supply.
More info on that PSU would help us out. Like sk1939 said, we need to know the power specs of that PSU. If going for a GTX 580, you need to have a much better PSU, that has at least one 8 pin and one 6 pin power connector.
No need to take a photo. I just gave you that picture to give an idea in your mind about where to find the power specifications on a PSU.
Just looks at the +12v rail and and just write down the amperes or 'A' on it.