pc_card_gameAccording to Nielsen [PDF], over 43 million people played 847 tracked casual PC games in March, 2009. 75% of the total minutes spent playing PC games is spent playing casual games. And 88% of the minutes spent playing casual PC games is spent playing card games.

Additional info: Dominant age bracket is 25-54. 58% are female (vs 37% for rpgs, 25% for shooters).

And heavy gamers tend to play twice as long in one sitting as casual gamers.