As someone who's using dual 5770's (A poor gamer at heart, and my machine reflects it!) I've found that it's not really worth the hardware investment at the moment with the massive difficulty jump to run the machine and the cards. That being said, I've found it worth while to let my machine chew away at mining while I'm /not/ gaming, and the machine is otherwise working, as I'm usually up coding or something, so it's certainly worth letting the cards churn away IMO.