Continue to support iOS 6.x ?

Hi, I'm about to start developing a new game. Now that iOS7 is out and drops support for iPhone 3GS, I'm wondering if I should set the minimum iOS requirement for my game to be iOS7?

This would mean that I would effectively be dropping support for the iPhone 3GS. Would that be foolish? Is the iPhone 3GS user base still large enough that I would be missing out on a significant amount of sales of my game?

Furthermore if I set the min to iOS7, then should I design all of my graphics to be optimized for a screen resolution of 640x960 (not bothering with 320x480 optimized graphics)?

Thanks