Here is info
http://www.microsoft.com/whdc/system...AE/PAEmem.mspx http://msdn2.microsoft.com/en-us/library/Aa366778.aspx
about memory limits in 32&64-bit and some memory tricks in 32-bit Windows systems.
And here
http://www.microsoft.com/windowsxp/u...loringx64.mspx
Charlie says exaclty, that 32-bit app need to be compiled with /3GB switch in order to be able to use additional gig of address space.
However, with regular 32 bit app you can have not big, but some advantage on a 64-bit system with more than 4 GB RAM by keeping many apps open simultaneously without swap them out. Even operating system itself + anti-viral programs et al may use 500MB, eating the memory available to your application.
What is bad about XP 64 Pro is its restriction of CPU using - it can not use more than 2 CPU, and I don't know how it will behave on 4-core CPU. In the end it's not too new version of OS.