**Please Note: These will all give a good number to go by(Read thier respective notes and instruction well), but usally each has different reading on what you need and only true way of knowing is to add up the wattage requirements of each component. Its is possible to have to little power but reasonably never too much **
On the 12v line of any power supply, make sure your Amp value is above 28a.
this is because your gt 250 video card requires a minimum 28a on the PSU, and 400watts.
Making headway for your intel CPU and hard drives, which all run off the 12 volt line I recommend a 550- 650watt PSU because it will supply enough AMP at the 12 volt line which is important to make sure you do not damage your hardware;
The specs for minimum power are grossly inflated generally so its best to do some legitimate thinking rather than just running off what the box says. The power ratings given are always for a system using a heavily OC'ed Intel Extreme Edition processor and are for the system as a whole.