MEMORY SIZES YOU CAN USE IN WINDOWS - ANSWER TO ALL WHO WONDER

You wouldn't waste money if you install 4GB REGARDLESS of Windows (starting
w/Vista) or hardware (starting w/post-2006) you have, but you can do more,
read my article:

As you know from Electrical Engineering basics, theoretical memory space is
defined by the address bus width. In a binary system it's simply 2^N where
N is the address width.
In plain English:
If your Windows is 32-bit it can theoretically manage 2^32 = 4GB.
If your hardware (e.g. Intel945 chipset?) is 32 bit but Windows is 64 bit,
or hardware is 64bit but Windows is 32 bit it's still 4GB as obviously the
lower address width limits the system, and disregards 64 bit elsewhere.

The practical numbers are somewhat different:

a) Windows 32-bit claims a whopping 0.75GB for itself ("untouchable" by
you - the user) due to I/O overhead in a 4GB memory space leaving you with
ONLY 3.25GB of usable memory
Too long to explain but just accept it as a fact - almost a Gigabyte is
eaten by the BIOS, I/O, etc. memory addresses your applications cannot use.

b) On the upside you can go over 4GB EVEN in a 32-bit Windows if this
Windows is a Server, and not a Client version.
There's a 99.9% chance you running a Client and not Server Windows, so your
usable memory is therefore:

3.25GB But I don't know exactly what Win version you run, so you can
calculate by understanding this article, i.e if you're so advanced as to
have Windows SERVER edition, then even in 32-bit configuration it can "page"
more than 4GB.

How much did you pay for 4GB and how much WOULD you pay for 3GB?

The $dollar difference as of September2009 is NOT worth any regret.
You'd lose pennies, stop worrying & have at least 4GB. Also better if you
install memory modules "symmetrically" which, as a consequence, also means
you'd have an even number of memory units.

In plain English, it's better to install two modules 2GB+2GB = 4GB, than
2GB+1GB = 3GB which is assymetric, although such may not longer be of any
importance for new computers, in the past it was an issue. Still I
recommend not to play with assymetry for DDR2 memory, I don't know about
DDR3 and this example covers 4GB. You might have 4GB+4GB=8GB for example,
for 64-bit Windows AND 64-bit hardware as a requirement.

Why would anyone need 8GB?

Well, I even need MORE - 16GB for CAD/Engineering & Graphics design work, so I can lots of memory running heavy-duty engineering simulations.

Plus it stimulates industry (but hurts environment)

So memory space is defined by 3 factors:

OS bit width

Hardware bitwidth

Whether your Windows addressing is using virtuial/paging/etc to bypass
normal addressing limted to 2^N - i.e. is it a regular Win Client or Server,
and how much your Windows wastes for itself (overhead)
Just get 4GB and be done with it.

BUT IF YOU NEED 1GB+1GB MODULES = 2GB of laptop, DDR667 speed memory, let me
know - I can ship for free (you just pay shipping by Post Office in USA or
abroad), I removed them and replaced with 8GB on my laptop.