This is found in SQL Server Books on-line. Initially we had 4 GB (3 GB for SQL + 1 GB for OS) with AWE Option disabled. Once we upgraded from 12 GB (10 GB for SQL + 2 GB for OS) we enabled AWE. It makes sense to enable AWE if we have more than 4 GB. Otherwise we will end up with some addressing issues. If you have less than 4 GB for SQL Server, please do not enable AWE option but you can install SP4. SP4 has a lot other performance and defects fixes, remember

Before enabling AWE, consider the following:

• When awe enabled is set to 1, instances of SQL Server 2000 do not dynamically manage the size of the address space. SQL Server will reserve and lock almost all available memory (or the value of max server memory if the option has been set) when the server is started. It is strongly recommended that you set a value for the max server memory option each time you enable AWE. Otherwise other applications or instances of SQL Server 2000 will have less than 128 MB of physical memory in which to run.
• If the total available memory is less than 3 GB, the instance of SQL Server 2000 will be started in non-AWE mode even if awe enabled is set to 1. In this situation, you do not need to manage AWE memory because dynamic memory allocation is used automatically.
• You can determine the amount of memory you can safely allocate to instances of SQL Server 2000 by identifying how much memory is available after all other applications to be used on the computer have been started. 

  Use the SQL Server Performance Monitor Total Server Memory (KB) counter to determine how much memory is allocated by the instance of SQL Server running in AWE mode. Configure the max server memory option to leave some additional memory free to allow for the varying needs of other applications and Windows 2000. For more information, see Monitoring Memory Usage.