400W is enough for most of PC´s, but with lots of modding and graphics card that need extra power supply u will need above 500W
i seen the new power supplies from 700W they look very nice. silenced and good for new dual cores.
The wattage of a power supply really means little. Yes there's a certain point where you need to a certain amount of watts. But you can have a 700 watt PSU that's useless if it cant handle the load on certain rails. That's where a high quality PSU comes into play.