Could you be any more vague? Any P4 chipset mobo and any P4?
There's really no such thing as a 'minimum' power supply. The CPU and motherboard aren't the only determining factors. The video card, hard drives, even case fans all help determine what power supply you'll need.
On top of that power supplies themselves can't be judged purely by how many watts they're rated at. What really matters is the output of each rail and how much each device in the PC will use from those rails. The watts themselves almost don't even matter.
What will happen if your power supply is inadiquate though is very simple. Your computer won't work.
If you're lucky the computer will just be unstable. It could spontaneously reboot and/or crash. Errors reported might come from just about any possible component, but memory seems to usually be one of the first to be indicated.
If you're unlucky your power supply will go up in a puff of smoke and may even damage components in your PC. (That's pretty darn rare though these days.)
"<i>Yeah, if you treat them like equals, it'll only encourage them to think they <b>ARE</b> your equals.</i>" - Thief from <A HREF="http://www.nuklearpower.com/daily.php?date=030603" target="_new">8-Bit Theater</A>