if you want to play fallout3 on top graphics then the hd4850 will do the job. The 8800 and 3xxx series amd cards are mid range but arnt much cheaper than the 4850. They should also do the job but you may have to turn a few settings down depending on other hardware.
Bare in mind you can pick up a good enough psu for about £50 (so im guessing about 60 dollars or so).
However it will be hard as a 300W PSU its pretty weak im afraid. Im not sure what you need even for a mid range card. I would recomend a 500W PSU even with mid range cards in this day and age.