I don't believe there is a direct relationship between RMS and PMPO. Acutally the PMPO rating is problaly just as useless as RMS. PMPO sure look like it is LOT more powerful than if it was rated in RMS. and I think the idea was to make you think you got more for your money.
Is 75w. enough? Well..maybe; it depends on the speaker system and how loud a person like his music.