...and if you really need lots of memory for applications you can use the /3GB switch in boot.ini to give the applications 3Gb of address space and the OS 1Gb.

(though this only works on the Windows XP, Windows Server 2003, Windows NT 4 Enterprise Edition, Windows 2000 Advanced Server or Datacenter Server and the application needs to know about it to get the extra address space)

http://www.sysinternals.com/ntw2k/info/bootini.shtml
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