Is it the GHz that makes a computer faster?  In simple terms tell me
about comparing a 3.0 GHz to a 2.0 GHz processor.

A 3GHz processor will execute instructions 50% faster than a 2GHz processor.

However, the speed of executing instructions is only one of the things
involved in overall performance, and some are of much greater
significance.  Chief among these, in my opinion, is processor
architecture, specifically, the instruction set.  The bare-bones
theoretical computer needs only one instruction - "decrement ans skip
on zero".  It could do that blindingly fast, but never be the basis
for a practical computer.  RISC processors perform a small set of very
simple instructions, like adding or subtracting, and can do so very
quickly, however, to perform a division may require a large number of
these simple instructions.  In contrast, a CISC machine, while it may
have a lower processing speed, may require only one instruction to
perform the same division.  Therefore, one cannot compare different
architectures just on processor speed.

Then there is memory access time, and the von Neumann bottleneck. 
Everything has to come and go to memory, and the speed at which that
can occur is very significant.  A fast processor may just sit idle for
longer waiting for a slow memory.

yadda yadda yadda

The real answer is - what you see is what you get.  Decide on your own
benchmark for performance and compare machines based on that, not on
the numbers.

Basically what xarqi said is true, but you also need to look at how
much RAM (also referred to generically as memory) is in the computer.
If you are looking at Windows XP, 256 is the minimum you needs, 512 is
good, and you need more for advanced video gaming (for gaming you also
need a good video card. An NVIDIA 6600GT or 6800 seem to be rather
popular these days).

The RAM is important because it will increase the APPARENT speed of a
computer. The processor (CPU) and bus speeds are what really drive the
computer, but without enough RAM your processor will not be able to
run at peak efficiency.

Also look at RAM speeds. DDR266 (pc2100) is the slowest DDR, then
DDR333 (pc2700), DDr400 (pc3200), and so on. The higher the number,
the better, but also the more expensive.