internal power supply by pass

is there any problem with feeding the mega board from and external atx supply to the plus 5vdc and ground bus thus bypassing the on board regulator (i want the 12vdc for motors only to reduce the rfi)?

After a little research it would appear that supplying a well regulated 5vdc to both vin and 5 vdc (pin3) on the power connector should take care of both the 5 and 3.3 volt needs. My only concern is IC 4, the mc 33269 5volt regulator. has anyone tried this and what are the potentials for overheating IC4? I intend to use an ATX server supply with a 2A stdby to feed the Mega an turn on the rest of the supply when the program comes up. I'm controlling 4 unipole steppers and want the drive power to come on after the processor is up and running so as not to have arrant motion.