The specifications for PC power supplies are at http://www.formfactors.org The regulation is typically 5%. In most new
PC's there is an on board regulator that powers the CPU to a tighter
tolerance. Some of the manufacturers produce supplies with tighter
tolerances.
One issue is the minimum loads that these supplies require to maintain
regulation. If you do not draw sufficient current the supply will go
out of regulation. It is not clear (at least to me) what happens if
the supply goes out of regulation. My concern was the output going
high and tripping an output voltage OVP which may require some sort
of reset.
When you design your supply don't forget to pull the PSON# signal low
to turn on the supply. PSON# is usually controlled by circuitry on the
motherboard. Use the +5VSB to power any circuitry you need to have on
all the time.
If you do not need a lot of power I think you are better off with a
wall-wart and some switching regulators.
Most of the PC power supplies are similiar. You could search the web. One such link where you could find some schematics & lots of useful stuff is www.smps.us (under computer power supplies)