would be good to know which CPU you have, those models of PC come with a variety of CPUs (although, not very strong ones unfortunetly)
off the top of my head, I think the 5xx series of nvidia were the last to support legacy. You should look at which motherboard you actually have. Even though it's built by HP.. it just means it's put together by HP. they may not necessarily have all available tools. you should look into the actual manufacturer of the motherboard for updates and UEFI support. I think that would be a better step.
but if that doesn't work. then you could just search for a GTX570 or higher... used. maybe you can get a good deal, but would performance increase be enough? that's another topic (if that's the only option you...