This is documented in the articles at http://support.microsoft.com
You can switch this to 3GB with the /3GB switch in BOOT.INI. Look up that?
Bob
My system information:
Windows XP Pro SP2
Pentium 4 CPU 3.00GHz
4GB RAM physically installed but system properties says 2.99GB installed
Behavior I am seeing:
Basically, after about 1.5 to 2 days of running from start up, my machine starts to have memory problems. Apps won't open at all or will start behaving strangly. I will sometimes even get a message to the effect "not enough resources to complete that action." Closing apps down seems to alleviate the problem.
Here's what sparks my question. I have 4GB of RAM installed. When it gets to a total commit charge of about 1.5GB, that's when it starts acting flaky. Shouldn't I be able to utilize more than 40% of my installed RAM?
Other information that may be useful: I am always running Outlook (with Franklin Covey PlanPlus v4), ActiveSync 4.5, Windows Desktop Search 3.01, an instance of Internet Explorer 7 with two tabs open and one instance of File Explorer. Then I go in and out of various applications for work. Sometmes I have 2 or 3 of those open at a time. The commit charge creeps up and up and up until it gets to 1.5GB and then I get strange behavior.
Am I having a problem or is this expected behavior?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic