The advent of processors breaking the gigahertz barrier has made one thing clear: their thirst for power is hard to quench. When buying a case, take a very close look at the built-in power supply, if included. In addition to the classic ATX power supply, it should also feature an auxiliary power connector. More and more motherboards require this plug to cover the additional power requirements of modern processors.
If possible, avoid plugging the monitor directly into the PC power supply. While some power supplies offer an additional plug for a monitor, I would not recommend using it. You will benefit more from the PC having as much power available to it as possible.
I would recommend at least a 600 watt PSU of a reputable brand, for normal PC use, some cheaper PSU’s cannot maintain the maximum power output for extended periods.