Its not the wattage that is important. Its the manufacturer. The generic ones will display a peak wattage and a peak amps on the rails. This means you are going to have to pay out for a good PSU. Even if it is a lower wattage, it is a guarenteed wattage, that it can maintain. Good companies are Antec, Thermaltake (these are very reasonable priced, but people who own one say they are great.) Enermax, Seasonic, and I just brought the Hiper type R (don't think it is readily avaliable in the US, but they are extremely popular in the UK.) And for the extreme overclocker, pc power & cooling.
And if you use a cheap one, it does
take out hardware. I have had a personal experience of this on friday at a friends house. I put my hdd in to get it formatted and to get xp on it, as my cd drive doesn't work anymore. And the harddrive was on fire! It also took out one of his cathodes, and his PSU now creates a bad smell.
With a good company this will not happen, as there is protection against it.
(Btw use this as a guide for a minimum wattage: http://www.jscustompcs.com/power_supply/
and as said above, 18a on the 12v rail is a minimum for a AMD 64 rig.