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.
I just read at HP's forum that even if I get a new power supply, my motherboard wouldn't support anything above 650 watt range. Is it true?
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.
I don't have a camera to get a picture of it. When my wife comes home I will do it.
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.
Here is the picture of the power supply
Well, it won't handle a GTX580. Get the TX750 like you had mentioned.
Best answer selected by dexterfly.
Also your motherboard can handle any wattage of PSU... As long as it has enough watts to run the system and as long as the current is 12 volts it is ok. ALL power supplies have 12 volts by the way.