Environment.Is64BitOperatingSystem is what you're looking for, check it out: http://msdn.microsoft.com/en-us/library/system.environment.is64bitoperatingsystem.aspx.

The .NET 4.0 framework now provides information in System.Environment Class.
System.Environment.Is64BitOperatingSystem
System.Environment.Is64BitProcess

Compiled as 32-bit running under WoW64 on:

Windows Server 2008/Windows Vista Version 6002 (Service Pack 2) MP (4 procs) Free x64
Product: WinNt, suite: SingleUserTS

Here's example GetSystemInfo() output:

0:000:x86> dt si
Local var @ 0x17f5cc Type _SYSTEM_INFO
   +0x000 dwOemId          : 0
   +0x000 wProcessorArchitecture : 0
   +0x002 wReserved        : 0
   +0x004 dwPageSize       : 0x1000
   +0x008 lpMinimumApplicationAddress : 0x00010000 
   +0x00c lpMaximumApplicationAddress : 0x7ffeffff 
   +0x010 dwActiveProcessorMask : 0xf
   +0x014 dwNumberOfProcessors : 4
   +0x018 dwProcessorType  : 0x24a
   +0x01c dwAllocationGranularity : 0x10000
   +0x020 wProcessorLevel  : 0xf
   +0x022 wProcessorRevision : 0x401

And GetNativeSystemInfo():

0:000:x86> dt si
Local var @ 0x17f5cc Type _SYSTEM_INFO
   +0x000 dwOemId          : 9
   +0x000 wProcessorArchitecture : 9
   +0x002 wReserved        : 0
   +0x004 dwPageSize       : 0x1000
   +0x008 lpMinimumApplicationAddress : 0x00010000 
   +0x00c lpMaximumApplicationAddress : 0xfffeffff 
   +0x010 dwActiveProcessorMask : 0xf
   +0x014 dwNumberOfProcessors : 4
   +0x018 dwProcessorType  : 0x21d8
   +0x01c dwAllocationGranularity : 0x10000
   +0x020 wProcessorLevel  : 0xf
   +0x022 wProcessorRevision : 0x401

On another CPU fields may vary.

Juanito

Private Const PROCESSOR_ARCHITECTURE_INTEL = 0
Private Const PROCESSOR_ARCHITECTURE_IA64 = 6
Private Const PROCESSOR_ARCHITECTURE_AMD64 = 9
Private Const PROCESSOR_ARCHITECTURE_UNKNOWN = &;;HFFFF

<StructLayout(LayoutKind.Sequential)> _
Public Structure SYSTEM_INFO
Dim wProcessorArchitecture As Short
Dim wReserved As Short
Dim dwPageSize As Integer
Dim lpMinimumApplicationAddress As IntPtr
Dim lpMaximumApplicationAddress As IntPtr
Dim dwActiveProcessorMask As IntPtr
Dim dwNumberOfProcessors As Integer
Dim dwProcessorType As Integer
Dim dwAllocationGranularity As Integer
Dim wProcessorLevel As Short
Dim wProcessorRevision As Short
End Structure


Declare Auto Sub GetNativeSystemInfo Lib "kernel32.dll" (ByRef lpSystemInfo As SYSTEM_INFO)

Public Shared Function IsPlatformX64() As Boolean
Dim sysInfo As SYSTEM_INFO
Dim osv As OperatingSystem
Dim fOk As Boolean = False
osv = Environment.OSVersion
If osv.Version.Major > 5 Or (osv.Version.Major = 5 And osv.Version.Minor >= 1) Then
GetNativeSystemInfo(sysInfo)
Select Case sysInfo.wProcessorArchitecture
Case PROCESSOR_ARCHITECTURE_IA64, PROCESSOR_ARCHITECTURE_AMD64
fOk = True
End Select
End If
Return fOk
End Function