When analyzing PC power consumption you have to look at load current of each voltage output. ATX Power supplies were originally balanced for an expected equipment load out. So much +5V, +12V, -12V, etc. so on average everything worked out and no voltage output was overloaded.
Then everything changed. HD makers stopped using the 12Vdc, USB became popular, low voltage processors and RAM became satndard, etc. Now almost everything comes out of the 5Vdc and 3.3Vdc outputs. The better, newer supplies take this into account and have heftier ratings on the +5 and +3.3 V outputs.
The el-cheapos call themselves 450W power supplies but don't supply any more of the needed power than a good brand 350W supply.
A loaded system (3gig cpu, 1gig ram, 2 HD, 2 optical drives, top end video) should have +5V @ 30 A and +3.3V @ 20 A. The rest of the voltage outputs don't matter much anymore because they aren't used. Note this only adds up to 216W!
I have a post somewhere in here where I go through the details of computing your power consumption for a real system. I'll try and find it.
Keep in mind the numbers may be changing again soon with Intel's CEO talking about "a 100w processor is reasonable".
Check out tomshardware they have a list of cpu consumption for all the different makes and models. The athlon types jump around between 55 and 75 W last time I checked. The 3000 may be up to 80w.