also the -j switch for make and jam takes only advantage of multi cores / multi cpus at compile time. not at run time. in other words, the higher the -j is set the more source files will be build parallel. a good number for -j is number of cores/cpus + 1, but you can even do more.
If you only provide -j it will automatically take as much processes as possible.
If the number is too high it will slow down compiling again.
On my gentoo box I figured -j7 is a good choice for everything, but best is you try it out yourself. compile something with "time make -j#" and increase # every time, don't forget to make clean before
