According to the documentation at http://msdn.microsoft.com/en-us/library/aa394102%28VS.85%29.aspx this property may assume the following string values:
"X86-based PC"
"MIPS-based PC"
"Alpha-based PC"
"Power PC"
"SH-x PC"
"StrongARM PC"
"64-bit Intel PC"
"64-bit Alpha PC"
"Unknown"
"X86-Nec98 PC"
Most of it is pretty obvious, but what is "X86-Nec98"? Is it some odd Japanese machine that requires a specialized version of Windows? Does it use a normal Intel-compatible processor?
"SH-x" is unknown to me also.
Edit: I'm developing a C# class to collect system information via WMI and I need to understand all possible returned values.
Many of those entries are for historical interest. Alpha support existed in NT 4 and NT 5 Betas but was dropped before the Windows 2000 name was introduced. Others reflect even older platforms.
If you are using Win32_ComputerSystem explicitly (rather than CIM_UnitaryComputerSystem or one of the other parent classes), you only need to deal with the architectures that Windows actually runs on today: x86, x64 & Itanium (add ARM if WinCE has WMI support).