generic

Hi there, I have a doubt. Can I give 12v to a GPU (turn it on) before I turn on the mainboard? I have a Gigabyte 7970, and a MSI Z77A-GD55, I also have a generic "500 Watt" PSU to supply the mainboard, and a HP DPS-1200FB PSU; I'm wondering if I can use it to supply only the GPU. Assuming...