It will be fine for medium+ settings. I'd just keep AA off and run at native resolutions. You won't win any awards or anything, but it will run any game you want.
I always like how people say "Light Gaming", which is doesn't pertain to the visuals at all. It refers to your gaming trends. A light gamer doesn't play much, as opposed to a Heavy game that play constantly. Most Heavy gamers I know don't even have that great of rigs. while some of the light gamers I know have top of the line stuff. I'm a light gamer and I like it to look GOOD, so I got the 4870. It's all what you are willing to spend and how good you want the visuals.... Sorry, I was being technical about it.