Check out [HTML5Boilerplate](http://www.html5boilerplate.com/ “HTML5 Boiler Plate”)’s responsive template stylesheet. It has some good default sizes. I added a 320px for the tiny iPod Touch screens (and my Razr Maxx’s vertical screen). With those sizes, I’ve had no issues on any screens I’ve tested on, including 1024*768.
Every project is a little different of course, but generally if it’s responsive, it’s probably best to let the content and the design dictate where the breakpoints are, and not so much any particular device’s screen resolution.
I mean, we should definitely make sure it looks good and functions as expected on all those devices, but I dont’ think it’s necessary that we base breakpoints on them.