I recently asked Antec (case) tech support this same question.
The CPU will make a big difference.
A "make or break" point with my PP-303XP (300W PS)
and an AMD CPU (according to them) would be around
1.5G (actual measurement).

I went out of my way to get an Athlon XP CPU since that "might"
use "up to" twenty percent less power since it is based on a
mobile design/core.   (Compared to "Classic Athlon", Duron,
or Thunderbird.)
Athlon XP 1700 - 1.47GHz (133x11.0)

A short answer would be there are no short answers...

               Rick Glazier

----- Original Message -----
From: "andre chiasson" <[log in to unmask]>
> Is there a specific way to determine when a 250 watt power supply is not
> enough?

                         PCBUILD's List Owners:
                      Bob Wright<[log in to unmask]>
                       Drew Dunn<[log in to unmask]>