It depends on the +12v rating you'll find on the PSU, nameplate. Not all 265W PSUs are created equal. And some rate their total wattage by peak power, not continuous power. It sounds like you may have a Dell Optiplex?
I presume the RX 460 you have in mind requires no aux 6-pin connection for power? If so, it should not require more than 75W from the +12V rail.