Once you determine the wattage your PC requires, you need to determine the efficiency you can afford. The PSU efficiency rating system all tends to be the same: 80 Plus.
Your PSU should have the following six ratings:
Bronze
Silver
Gold
Platinum
Titanium
Power supplies with 80 Plus Titanium certification are more efficient than those with Bronze certification, meaning they consume less energy (heat) during AC to DC conversion. Three load levels are typically measured: 20%, 50%, and 100%. Despite being rated at 50% efficiency, Titanium power supplies function just as well, if not better, during heavy loads.
The internal components of an efficient machine are also likely to be subjected to less heat and last longer. The higher a power supply's certification, the more reliable it tends to be. The warranty offered by most manufacturers is a plus as well.