Amperage?

I hve a breaker (switch) box in out apartment that has the single line switches marked as being 20 amps. The main breaker switch (the one that switches all of the lines at once) however is marked 15 amps. The wall outlets are 20amp Pro models made by Levitron (which I will be replacing as soon as I figure out if they should be 15 or 20 amp). My question is should I be using 15 amp power cords or 20 amp power cords, or, does it make a difference (my amps have 15 amp IEC connectors, I think), also what about the outlets themselves (15 or 20 amp)? I do not own or intend to own high power or current amps. I owm an SET and a Musical Fidelity 50 watt amp.