Firstly, your power supply is way to less to run a low-mid end gaming system.
if your budget is tight change it to 500 watt and above and you will be alright.
secondly.. your cpu, mobo and ram need an upgrade.. cause they will bottleneck the graphic card that you are going to buy.. (9800gt i think wont.)
Thirdly.. if you are running at high resolution get the 550ti or 650ti will sure be a good choice .
(sorry for my bad english hahass) hope it helps!