It depend if you are doing light gaming or full overall performance upgrade.
If you are gaming upgrade Ram to 4gb (35$) make sure it is the same kind you are running that will fit in mobo DDR2 or DDR3
and a radeon 7750 (100$)
For a better upgrade replace the mobo 80$ with an AM3+ socket and a processor of the fx series(4300, 6300) 110$.