One of the interesting features of the AMD64 X2 architecture is that its declared power consumption is not higher than of single core AMD64 processors running on similar frequencies. So if you perform significant amount of tasks that can be parallelized (e.g. running a build daemon), you can choose between two different advantages of the dual core system: higher performance or reduced power consumption.
The latter should be achievable by running the CPU on half of its standard frequency (using the Cool`n’Quiet technology). This way you may get a high performance system that can be easily dynamically turned into a system with about half the CPU power consumption and performance of a single core system running on the full CPU frequency. Cool. It seems it makes perfect sense to run ‘nice make -j3′ on a dual core system.