If you use 32 bit Windows, it will recognize just over 3 gigs of memory, but not the full 4. It will recognize even less if you have a graphics card that uses a ton of memory as well.
My suggestion would be to see if your computer can handle higher speed memory and keep it at 2 gigs. Say you have 2 gigs running at 533mz, see if you can find compatible, higher performance memory that runs at 800 in dual channel.
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