Which 500W PSU? They are not all created equal. And there's no problem with the card actually fitting. If there is (1) enough room for the length & width of the card, and (2) the motherboard has a PCIe X16 slot, it will fit. Since you already have a gfx card in the slot and the card you are buying isn't any longer or wider, of course it will fit.
But if you are asking if it will WORK, that is a different question. Yes, it should. The only reason it would not is (1) if the motherboard BIOS is so old it doesn't recognize the newer cards. Not likely, but it sometimes happens with OEM PCs. And (2) if your PSU is too weak or cheap.