Having a 64bit server operating system will greatly enhance your server and network performance. 64bit means you can install more than 4GB on your machine. In contrast, 32bit only allows you to install up to 4GB as a maximum. Keep in mind that with 64 bit, you’ll need to make sure all of your server software is compatible with the 64bit operating system.
If you’re running a Windows 2003 Standard Edition Server that is 64bit, then your 
Exchange software must also be running 64bit. This means that you cannot run Microsoft Exchange 2003 since it is only 32bit. You will need to purchase Microsoft Exchange 2007 which is 64bit and can run with Windows 2003 Server 64bit.
This is helpful information if you’re planning to buy or upgrade your server.
