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For testing purposes, I'd like to make an PC power supply operate when
NOT installed in a PC. I've heard that the AT supply needs at least
SOME load on its 5-volt output before it will operate; is an ampere
or two sufficient?

For ATX supplies, I visited a friendly computer-repair store today and
examined the "cheater" into which they plugged UNconnected ATX supplies;
it had three pairs of connections:
1) A jumper from Ground to PS-ON
2) an LED and resistor from PowerOK to Ground
3) two wires from +5V and ground going into something about the size
of a small box of matches which was shrink-wrapped to the cable

They wouldn't let me disassemble it !-)
Was there probably a 5-ohm resistor in that box?

Most all computer switching supplies need some load on it to start. About
1/2 to 1 amp should be fine. YOu can use a 5 ohm resistor rated at 10 watts
or so to put a load on the 5 volt line. When testing an AT supply I usually
just hook up to an old hard drive.
If you have some kind of load already hooked to the supply they you can
forget the resistor load.