Well, the cheapest option is to get a HD7750. That card needs no auxiliary power, and will run on your 300W PSU. It will play any game, but new ones will need to be turned down, possibly quite a bit.
Your other option would be to spend $39 on an Antec VP-450, which can easily power a HD7850. That combination may just exceed your budget, but a GTX650Ti (~$140-$150) should be able to squeeze within it.