You need a 64 bit processor in order to install the 64 bit version of Windows Server 2003 Standard.
Windows Server 2003 Standard x64 Edition is well-suited for High-Performance Computing (HPC) clusters, Terminal Services, Active Directory data stores larger than 2 GB, databases, business applications, Internet Information Services (IIS) 6.0, and memory-bound applications that do not require more than four processors. Windows Server 2003 Standard x64 Edition supports up to 32 GB of RAM on 1- to 4-way servers. Supported processors include: AMD Opteron Intel Xeon with Intel EM64T Intel Pentium with Intel EM64T Windows Server 2003 x64 provides high performance for both 32-bit and 64-bit applications on the same system. The underlying architecture is based on 64-bit extensions to the industry-standard x86 instruction set, allowing today's 32-bit applications to run natively on x64 processors such as the AMD Opteron and the Intel Xeon with Intel Extended Memory 64 Technology (EM64T). At the same time, new 64-bit applications are uted in 64-bit mode, which processes more data per clock cycle, allows greater access to memory, and speeds numeric calculations. The end result is a platform that leverages the existing wealth of 32-bit applications while also providing a smooth migration path to 64-bit computing