[dec]

Hola,

En cierta unidad de la JVCL se implementa lo que necesitas, empero, he aislado el siguiente código, de modo que puede usarse la función "IsWindows64" sin más:

Código Delphi [-]
const
  PROCESSOR_ARCHITECTURE_AMD64 = 9;
  {$EXTERNALSYM PROCESSOR_ARCHITECTURE_AMD64}
  PROCESSOR_ARCHITECTURE_IA64 = 6;
  {$EXTERNALSYM PROCESSOR_ARCHITECTURE_IA64}

function GetNativeSystemInfo(var SystemInfo: TSystemInfo): Boolean;
type
  TGetNativeSystemInfo = procedure (var SystemInfo: TSystemInfo) stdcall;
var
  LibraryHandle: HMODULE;
  _GetNativeSystemInfo: TGetNativeSystemInfo;
begin
  Result := False;
  LibraryHandle := GetModuleHandle(kernel32);

  if LibraryHandle <> 0 then
  begin
    _GetNativeSystemInfo := GetProcAddress(
      LibraryHandle, 'GetNativeSystemInfo');
    if Assigned(_GetNativeSystemInfo) then
    begin
      _GetNativeSystemInfo(SystemInfo);
      Result := True;
    end
    else
      GetSystemInfo(SystemInfo);
  end
  else
    GetSystemInfo(SystemInfo);
end;

function IsWindows64: Boolean;
var
  ASystemInfo: TSystemInfo;
begin
  GetNativeSystemInfo(ASystemInfo);
  Result := ASystemInfo.wProcessorArchitecture in
    [PROCESSOR_ARCHITECTURE_IA64, PROCESSOR_ARCHITECTURE_AMD64];
end;

A ver si te apaña o qué.