If there is one component you shouldn't skimp on, it is the PSU. A PSU which goes bad can take out a bunch of expensive components like your motherboard and CPU.
Some general guidelines -
(1) The rated wattage can be misleading. Some manufacturers use peak wattage whereas the correct one should be average wattage.
(2) Some manufacturers get the wattage by adding up the various rails. The rail you should pay attention to is the +12V rail. Around 36 to 40A on the +12V rail should be the minimum.
(3) Only buy from reputable brands and avoid buying second hand. PSUs deteriorate over time and it is not worth saving a few dollars and risk a bad PSU taking out your system.
(4) Read the reviews on Newegg or if available, on reputable tech sites like Anandtech.