If you're looking to get your power supply to run at its highest efficiency, usually around 40% ~ 60%, then get a power supply that has about double the amount of the power your system takes at peak, though unnecessary since the efficiency difference won't be justified with the extra cost that is incurred.

To prevent any possible overloading scenarios, then about 100W should be enough under peak. If you want to be even safer then go with larger capacity power supplies for more headroom.