You would need to do an in-place upgrade to Windows 2003 Enterprise Edition SP2/R2 x86, which can use up to 64 GB RAM.
http://msdn.microsoft.com/en-us/library/aa366778(VS.85).aspx#physical_memory_limits_windows_server_2003
See the SQL 2005 Books Online for important information about enabling support for > 4 GB RAM:
To enable Address Windowing Extensions (AWE) for Microsoft SQL Server, you must run the SQL Server 2005 Database Engine under a Microsoft Windows account that has been assigned the Lock Pages in Memory option and use sp_configure to set the awe enabled option to 1. The Maximize data throughput for network application option of Network Connection in Control Panel must be selected.