If you are gonna overclock it then no way..I have an overclocked PIII, and I noticed a big difference when I put in my enermax 431..before it was unstable, and my fan speeds especialy on my Alpha P3-125 were wildly erratic, and I couldn't clock as high as I can now.
A 250 watt P.S. is only good enough if everything is "onboard" video lan, sound, etc, and I would still expect instability from a small P.S.
For overclocking a P4 or Athlon with a juicy vid card, I would go no lower than 450w.
My fan speeds went from 4890 to 5114rpm...also the 450 watters usualy have two fans instead of one, or at least mine does...and the Enermax 431 has the all important 12v plug needed for the P4.