How do you get total amount of RAM the computer has?

Add a reference to Microsoft.VisualBasic and a using Microsoft.VisualBasic.Devices;.

The ComputerInfo class has all the information that you need.

The Windows API function GlobalMemoryStatusEx can be called with p/invoke:

  [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
  private class MEMORYSTATUSEX
  {
     public uint dwLength;
     public uint dwMemoryLoad;
     public ulong ullTotalPhys;
     public ulong ullAvailPhys;
     public ulong ullTotalPageFile;
     public ulong ullAvailPageFile;
     public ulong ullTotalVirtual;
     public ulong ullAvailVirtual;
     public ulong ullAvailExtendedVirtual;
     public MEMORYSTATUSEX()
     {
        this.dwLength = (uint)Marshal.SizeOf(typeof(NativeMethods.MEMORYSTATUSEX));
     }
  }


  [return: MarshalAs(UnmanagedType.Bool)]
  [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
  static extern bool GlobalMemoryStatusEx([In, Out] MEMORYSTATUSEX lpBuffer);

Then use like:

ulong installedMemory;
MEMORYSTATUSEX memStatus = new MEMORYSTATUSEX();
if( GlobalMemoryStatusEx( memStatus))
{ 
   installedMemory = memStatus.ullTotalPhys;
}

Or you can use WMI (managed but slower) to query TotalPhysicalMemory in the Win32_ComputerSystem class.

Add a reference to Microsoft.VisualBasic.dll, as someone mentioned above. Then getting total physical memory is as simple as this (yes, I tested it):

static ulong GetTotalMemoryInBytes()
{
    return new Microsoft.VisualBasic.Devices.ComputerInfo().TotalPhysicalMemory;
}