I just handed off a 90nm Sempron64 2800+ to a customer. I had it undervolted to 1.1v and it was completely stable (Prime95 and 3DMark). Best of all, I think it would be relatively easy to passively cool. In fact, I had it sitting on my bench with Prime95 running on the stock heatsink and the fan unplugged! It ran for 1 hour before the heatsink got hot (~50C) and I aborted the test. Being that the whole setup was on my work bench with no airflow and the stock heatsink (and thermal pad), I think it could be very easily cooled passively. The Sempron64 2800+ was only $75 USD on Newegg.

That's great to hear because I'm actually planning to upgrade this box with a Sempron 64 2800+ in order to do some x64 OS testing...

By the way, the new Morex 120W PSU is working great, I think I might have killed the other one by producing a short-circuit. And I managed to reduce the CPU temperatures by adding one 80mm fan @ ~1000RPM which is very quiet. I haven't done any undervolting experiments yet (and I probably won't do since upgrading to a Sempron64 implies upgrading the mainboard) but the temperatures now stay below 40°C at full load

Thanks
lowpowercomputing