It's wattage, not MHz.
Check out <A HREF="http://www.pcpowerandcooling.com/maxpc/cases.htm" target="_new">this</A> chart for a good idea on what's needed. Remember to take your total and divide it by 7 and then multiply it by 10, so as not to put too much strain on the PSU.
<b><font color=blue>~ What do you mean "It isn't working!"...Now where's my sonic screwdriver? ~ </font color=blue></b>