I built this for my girlfriends mom not too long ago. She doesn't game, or have any intentions of overclocking. Just over 500$. Coming from experience, Intel HD4600 is pretty dang decent. I feel like it would have to be able to run second life. You probably don't really need a GPU. If you want to spend the extra money anyway you could get one though. Also you should put an SSD in it.
This is pretty solid. Maybe a cheaper case. --- god invented extension cords. -elchris79