How to know total speed of desktop pentium 4 @1. 6

If you have a single core 1.6 GHz P4, your system is too slow (old) to meet either requirements.

The 2.4 GHz requirement is if you have a single core P4. The second requirement is lower because it requires two cores to do the work.

What software?

Its also probably a reference to the jump from the netburst (P4 and Pentium D) to the Core architecture (Core 2 Duo) which is much more efficient per hz. For instance my single core Celeron 430 system at 1.8 GHz is much much faster than my old P4 @ 2.4 Ghz.

If its older software, its highly unlikely to take advantage of multiple cores.

If its a 3D PC game, your video card might be much more important.