|Your Hp Pavilion H8-1213c has an ATX form-factor motherboard, so any ATX PSU will fit inside it since ATX is a worldwide standard which defines the size and dimensions of the motherboard and the PSU.|
The only thing that you need to make sure of is that the PSU you buy has SATA connectors for the internal drives in your PC, and all new PSUs have those anyway.
You will need to buy a PSU which has an output wattage of 600-watts minimum for either of those cards, to allow for the power required by the other components in your PC. I would play it safe and get a 700-watt PSU to be sure that there's enough power for everything without causing the PSU to be working flat out all the time. The PSU will also have a longer life if it's not working at it's maximum output capacity.