You're right to consider the capacity of the outlet. Bear in mind a couple of other things.
1. Whatever the maximum wattage rating of the PSU, that is NET power available to the computer. But the PSU has an Efficiency rating, which is the % of power consumed actually available. A good PSU will have 80% or slightly better efficiency. One rated for 1000W would actually consume from the wall outlet 1250 watts IF the computer were actually using all 1000W available.
2. Consider also any other device plugged into the same circuit, such as printers, speaker amps, monitors, etc. And I said "circuit", not just "outlet". Many circuits have more than one outlet connected into them. A "circuit" is one power line fed from a single breaker (or fuse) in the main panel. The circuit's limit, as MightEMatt said, is set by the circuit breaker (or fuse) rating.
3. In designing circuit wiring for a house, a rough rule of thumb used is that the planned use of the circuit (its regular load) should not exceed 80% of the circuit breaker rating. So, one would plan that a 20A circuit should normally have a maximum steady load of 16 amps. Now, that is NOT a strict requirement. It is used mainly to avoid having the breaker trip off unnecessarily because of some transient surge when it is already operating near it limit. But the breaker AND the wires in the walls are designed to operate continuously and safely very close to the amp limit stated. In fact, most breakers will not trip right at their rated limit unless the load is kept there for a long time.