Lookig for a new video card for my p6202f

Hi im currently using a HP p6202f desktop with a poorly onboard gpu. I'm looking video card that could handle game at max setting such as DeadSpace,MW2, or Assassin Creed2. Or maybe future game such as Mass Effect 2, and StarCraft2. But im not sure which video card to get that would be strong enough and compatible with my power supply. I have on my mind a 9800GT but im not sure how much vattage that GPU requires and how much wattage my PSU ca handle.

Some help would be nice, Thanks.