Mouseposé works best on Mac OS X Leopard (10.5.x), but should work fine on most Macs running OS X Tiger (10.4.0) and higher. It is usually being developed and tested on the current releases of Mac OS X. Mouseposé 3 will definitely not work on any version of Mac OS X 10.3 Panther.
The Window Focus feature and it's settings are only available on Mac OS X Leopard!
In addition, Mouseposé needs a graphics card capable of running Quartz Extreme technology. This should be the case with any machine that decently runs Tiger. Mouseposé does support multi-monitor configurations.
In any case, please try and test Mouseposé before buying a license. The time-limited demo provides all features and is technologically the same as the licensed version. If the demo will not run on your machine, buying a license won't help.