How choose MPU for image streaming and transmission
camera sensor of 5Mpx, i want to elaborate video from this sensor and stream it through a wifi module [...]
  use an integrated module Cortex-M3 120MHz 128KB RAM with wifi


The 5MPx picture requires 5 MegaBytes (!) of RAM to be stored only.
Thus you need much beefier equipment. Video compression is really computational expensive.

I would strongy suggest using some ARM SoC board with Linux/Android, like the ones inside mobile phones. Look for somthing that fits your camera sensor's interface.