64bit systems get rid of the limits of the 32bit operating system.
More Memory, Better performance

Lets start with the numbers
32bit Processor/Windows
-4,294,967,295

64bit Processor/Windows
-18,446,744,073,709,551,615

TO take full advantage of the 64bit architecture you need at least 4GBs of RAM

It is best to get the Operating System that goes with your processor as if you purchase a 32bit version of windows your entire operating system will be capped at the potential of the 32big architecture and will be half utilized and you will not be able to use your system to it's fullest potential

Instead of being limited to 4GB of RAM you will then be limited to 16TB of RAM


http://msdn2.microsoft.com/en-us/net.../aa496329.aspx
http://www.microsoft.com/windowsxp/64bit/overview.mspx






Some good reading
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If you're looking for large amounts of memory and improved floating-point performance for tasks that rely on real-number data, such as mechanical design and analysis, three-dimensional (3-D) animation, video editing and composition, and scientific and high-performance computing, Windows XP Professional x64 Edition is designed for you. With its 64-bit processor, Windows XP Professional x64 Edition gives you access to greater amounts of memory while continuing to support 32-bit applications.
More memory, improved performance

The expanding data and performance needs of business, academic, engineering, and scientific organizations have pushed the limits and capabilities of existing information technology (IT) platforms. Millions of people worldwide need to access gigabytes or even terabytes of data in real time. The increasingly sophisticated demands for making home movies, working with digital photographs, using digital media, and playing 3-D games is also stretching the capabilities of existing 32-bit PCs.

Advances in processor technology that extend the capabilities of x86 architecture have brought the power of 64-bit computing to you. Now, you can use a 64-bit operating system to seamlessly run 32-bit and cutting-edge 64-bit applications. Windows XP Professional x64 Edition takes full advantage of this new architecture.
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Comparing 32-bit and 64-bit editions of Windows

The major difference between 32-bit and 64-bit editions of Windows is memory support. With more memory, you can run more programs at the same time, and run more memory-intensive programs like multimedia, gaming, and database programs. And your computer's performance will be faster overall.

Currently, 32-bit editions of Windows are capable of supporting up to 4 gigabytes (GB) of system memory, with up to 2 GB of dedicated memory per process. Windows Professional XP Edition x64 currently supports up to 128 GB of RAM, with the potential to support up to 16 terabytes of virtual memory as hardware capabilities and memory sizes improve.

Another difference between 32-bit and 64-bit editions of Windows is the increased number of registers available on 64-bit processors. This enables faster performance for applications that perform intensive floating-point calculations.

Windows XP Professional x64 Edition provides faster performance, increased reliability, and greater flexibility to today's most demanding users. With Windows XP Professional x64 Edition, you can take advantage of the new 64-bit technology to improve your PC or workstation performance.

http://www.microsoft.com/windowsxp/64bit/overview.mspx
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