http://msdn.microsoft.com/en-us/windows/hardware/gg487503.aspx refers to XP and other os ram limits.
Below is my boot.ini file
boot loader]
timeout=5
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /3GB /fastdetect /noguiboot
Afaik, they dropped the PAE after sp2 (or maybe it was sp1 - I'm on sp3). noexecute implies it any way.
Other things, set the swap/page file to a disk other than the the OS, if possible. You can get there by: - my computer, system properties, advanced, performance settings, advanced, virtual memory, change . I've actually allocated an 8GB partition to the paging file, with an initial size of 4000MB as a custom size. Others may argue about letting the system manage the size - I'm just saying what works for me. Also set the memory usage and processor scheduling to 'programs' (on the last 'advanced' window).
You may also need to change your bios settings. If you still have less memory than you think you should have available, then I would suggest you do the three fingered salute, and check what processes you have running. Some software does not close properly, and still leaves memory allocated. Some devices may also use shared memory, but hopefully not many these days.
hth.
Best wishes,
Ray