I find these are good breakpoints to start from but always test and tweak as you go. I'd also suggest using ems instead of px for dimensions for varied device dimensions and resolutions (reasons described here (http://blog.cloudfour.com/the-ems-have-it-proportional-media-queries-ftw/))
So the above queries would look like this:
@media (min-width:20em) { /* smartphones, iPhone, portrait 480x320 phones */ }
@media (min-width:30.063em) { /* portrait e-readers (Nook/Kindle), smaller tablets @ 600 or @ 640 wide. */ }
@media (min-width:40.063em) { /* portrait tablets, portrait iPad, landscape e-readers, landscape 800x480 or 854x480 phones */ }
@media (min-width:60.063em) { /* tablet, landscape iPad, lo-res laptops ands desktops */ }
@media (min-width:64.063em) { /* big landscape tablets, laptops, and desktops */ }
@media (min-width:80.063em) { /* hi-res laptops and desktops */ }
There is also a nifty pixel to em calculator online here (http://pxtoem.com/)
For those of you not as familiar, including myself.
