Thanks guys - I am currently undervolting my athlon (using standard
133x15)
and I was trying to figure out if dropping the volts from 1.65 ro 1.5
would
actually generate significantly less heat or not!

1.5/1.65=0.909, so you can figure on about an 11% reduction in heat.

I figure that at 2000Mhz power usage is (as above) 68.3W, but dropping
from
1.65v to 1.5v reduces power intake to 56.4W - 17.5% less power, therefore
17.5% less heat generated. Does this sound reasonable?

It does. Not that the two numbers don't match up (11% vs. 17.5%) because
the 17.5% number obtained by comparing the theoretical maximum value to a
real-world value. Had you measured the real-world usage before and after,
the difference should be much closer to 11%.

If you really wanted lower power, you could go with a mobile Athlon XP,
where the max power draws can be as low as 35 watts, and the "power now"
featured (I believe there's at least one desktop board that supports it)
will keep actual draws much, much lower when not under load. However, it
does sound like you've done an outstanding job with what you've got!

steve