A few things.
GHz is a measurement of one billion clock cycles per second. How much work is done per clock cycle? Depends on the type of processor.
You can be sure, for example, that every Pentium 4 does the same amount of calculations per clock cycle as every other Pentium 4. So GHz is fine when comparing two Pentium 4's. But when comparing a Pentium 4 to an Athlon 64, or a Pentium 4 to a G5 in a Mac, it's almost useless.
How do you get real performance?
It depends on:
1) The GHz (clock speed).
2) The work done per clock cycle (depends on the type of processor)
3) The bus speed
4) The size of the cache
5) The speed of the RAM
6) The amount of RAM
7) How well the software is written
and many other factors.
Frankly, it's almost impossible to compare a Mac to a PC.
The PowerPC processors in Macs have a lower clock speed, do more work per clock cycle, have a higher bus speed, and the OS is much better optimized because it only needs to run on Apple hardware. So just try the machine and see how it feels.