Im guessing its an old prebuilt machine, saying that the PSU is mosyt likely a poor quality unit, meaning that you would also have to acquire a new PSU to get a card.
Tell me what games/software you want to run, if your not really looking for anything to fancy you can buy an older gen card, wich is still good and a decent PSU for maybe around 100$ (estimating that budget)
If you are looking to play some newer games you need at least 100$ for the GPU and 50$ for a PSU.
If you tell me how much you want to spend i can try and find something. Also tell me were are you from.