A rather comprehensive answer from comreviews.com:
TLDR:
If you are intending to use a faster
memory module in your computer, here
is a summary of the items to consider
before purchasing and installing it:
- The memory has to be of the same
technology (DDR, DDR2 and DDR3 are not
cross compatible)
- The PC must support
the memory module densities being
considered
- No unsupported features
like ECC must be present on the module
- The memory will only be as fast as
that supported by the memory or as
slow as the slowest installed memory
module
The answer to this question really is
yes and no. If you are talking about a
computer for example that uses DDR and
you want to use DDR2, it will not
work. They use two different clocking
technologies that are not compatible
within a system. Now, if you are
looking at the same type of memory as
what is already in the system, then
you can buy memory that is faster than
the existing memory. Of course, there
are always restrictions.
In order for this to work, the memory
modules must be of a density that is
supported by the computer motherboard.
For example, a system that can support
1GB modules cannot use a faster 2GB
memory module as the system will not
recognize it. Similarly, if your
motherboard won't support memory with
ECC, it won't support faster modules
if they have this feature.
The other issue has to do with the
speed. Even though they may be faster
modules, they will not be running at
the faster speed. This can happen in
two cases. The first is that the
motherboard or processor will not
support the faster memory speed. When
this happens, the modules will instead
be clocked at the fastest speed that
they can support. If the modules are
installed with some slower ones
already installed, the memory system
will run at the slowest memory timings
and speed.
So, why would you want to install
faster memory in a system if it will
still run at the slower speed? It has
to do with availability and pricing.
Often times as memory technology ages,
slower modules may drop out of
production leaving only faster ones
available. Such might be the case with
a system that supports PC2700 DDR
memory but all you can find is PC3200
DDR modules. Memory is considered a
commodity and as a result has variable
pricing. This can result in situations
where a faster memory module might be
less expensive than a slower one. If
PC2-5400 DDR2 supplies are tight, it
might be less expensive to buy a
PC2-6400 DDR2 module instead.
