what happens if the psu isn't powerful enough?

System instability, random reboots, PSU runnning very hot among other things.

It leads to a number problems system rest @ any time. Can burn out the power supply. OR any otherpart in a computer. It can also rest if you start useing an inactive device like a cd drive or a floppy or a usb powered device. Or if you double up on cables like 3 to 4 drive hard drives things like that when your power supply does have enought power to power more things. This you can tell if your system has all ways had power and as soon as plug in hardware it does rest @anytime. you have tell more information about the computer so I could give a better answer. also system can run hotter if you fell the power supply fan. Also system instable.