Hi, I came here to ask a similar question. I'd like to connect a PC PCU (AT or ATX) to my A500, too.
Apart from the fact that I never enjoyed any apprenticeships in electrotechnics and therefore I always confuse stuff like Volt, Ampere, Ohm ect, I assume that the amount of Ampere is also very important (not just connecting the +5, -5, +12, -12 and GRD cables properly). As far as I know an AT or ATX PSUs got a bit more Ampere... or am I wrong?
Maybe someone knows a bit more about this kind of magic, before I send my A500 straight to the pearly gates of Amiga Heaven...
If you look at the link I gave above, you will see that hooking up a AT-PC style PSU is good and sound/safe...
I have put my A1200 (uses the exact same PSU as a A500, less wattage) into a Tower case using a 300W AT PSU, to power the MOBO, 3 HD's, CD-ROM, and a Floppy... and it is kicking without any problem...