A human engine has a torque curve similar to that of a steam engine, more or less, which is to say that it is flat until a certain critical rpm, at which point it begins to drop off, because the energy used in accelerating and decelerating massy components (legs) begins to take up all the energy produced.
Asterisk is known to run on many OS platforms. However, Linux is the main platform for development and Digium hardware support. If you are running VoIP only, or if you are comfortable with using external media gateways to connect conventional telephone equipment, then you have more systems to choose from, like FreeBSD, Mac OS X and Solaris.
Asterisk is a complete PBX in software. It runs on Linux, BSD, Windows (emulated) and OS X and provides all of the features you would expect from a PBX and more. Asterisk does voice over IP in four protocols, and can interoperate with almost all standards-based telephony equipment using relatively inexpensive hardware.