How do I know what kind of graphics card upgrade I need to play most pc games?
9800gt can play most games on the market at reasonable settings. A 9600gt will run most games fine but you going to be missing a lot of the details in some of the "prettier" games.

You may need to upgrade your power supply as well.

Please tell us what computer you have, what resolution your monitor is and what budget you have in mind for further help