Agree with Conch_Pearl. If yours falls over then fit a 450/500w one and all will be well. It may be a while though, as even the little 250w ones will survive a while if you are not using your peripherals intensively.
You should be able to hang a fair few number off. USB peripherals don't take much load, so must be preferred to peripherals that take direct AC, which some like monitors do on some sytems. Very difficult to give a number though, unless you give details of system.
With a 500w PSU wattage won't be as much of a problem as heat. We had to upgrade with dual fan PSU's plus bigger heat sinks and fans on the CPU's on some of our servers that run 24/7.
The point is peripherals will be running even when not in use driving up the temperature.
You might get away with it, but I suggest you take a look at the running temps. in the bios after running for a while.