39472: "It may not be worth running BOINC on old computers because the science completed by an old 450MHz desktop may not outweigh the expenditure of electricity and thus fossil fuels. Is the tiny amount of science crunched by old computers worth the extra pollution?"

Great point. More generally, at what RAC does a dedicated host become environmentally justifiable? Maximizing a dedicated host's RAC/watt can involve more than getting the most power-efficient CPU - unplugging the monitor, keyboard, mouse and video card (assuming the host isn't being used) will save a good amount of energy.

And though I haven't experimented with it yet, BOINCpe supposedly makes for more energy efficient computing because it uses a CD-ROM drive instead of the typical hard disk configuration. I would be interested to know the average energy savings from switching to CD-ROM based BOINCpe. The only way I can think of to easily measure the difference is with a power meter, something like a P3 P4400 Kill A Watt Electric Power Usage Monitor.