Wattage is overhyped but it can be useful, especially if you are running alot of stuff in your case.
My understanding is that you need these specs to run an AMD xp:
+5V current should quote 35 to 45 amps (45A)
+12v current should quote 18 amps or more (18A)
+3.3V current should quote 28 amps or more (28A)
I found this at a site that I linked above.
Of course, if the power supply meets the minimum requirements then the chip will run fine, but you may not be able to overclock, if that is what you want to do.