Why is it overkill? The PSU I'm prospecting has 37A on +12V. It
s better to have more power than less so might as well get a higher A on the 12v. 20A seems a bit low, and a little risky, better to be safe.
Unless he's running 2 X850XT PE's with atleast a gb of RAM, couple 10k hard drives, couple optical drives, heavily OC'd CPU, then he isn't going to need more than 20a and can save some money