Interestingly enough, I just bought a budget Asus i3-3120M laptop ($350) with no discrete graphics (just Intel HD), 4GB RAM and it cast chrome tab at 720p perfectly. Yet I can't do it on my i5 or i7 laptop and both got 8GB of RAM, and faster cpu benchmark. Both costed 3 times more than the Asus i3. They are all on the same wireless network and using intel wireless adapters. The only thing I could think of is that the i3 is running Windows 8 and both the i5 and i7 are running Windows 7 x64. The other thing is that the i3 is ivy bridge and the i5 and i7 are sandy bridge.
Macbook Pro 15 with i7 can do 720p. I haven't try it on my Macbook 13 yet but it is a few years old and running Windows XP, so I don't think I really want to try.