Find percentage used physical memory - with aid of WMI code generator http://www.microsoft.com/en-ie/download/details.aspx?id=8572

gistfile1.cs

using System;
using System.Management;

namespace WMIQuery
{
    public class UsedPhysicalMemoryQuery
    {
        public static void Main()
        {
            try
            {
                ManagementObjectSearcher searcher =
                    new ManagementObjectSearcher("root\\CIMV2",
                    "SELECT * FROM Win32_OperatingSystem");

                foreach (ManagementObject queryObj in searcher.Get())
                {
                    double free = Double.Parse(queryObj["FreePhysicalMemory"].ToString());
                    double total = Double.Parse(queryObj["TotalVisibleMemorySize"].ToString());
                    Console.WriteLine("Percentage used: {0}%", Math.Round(((total - free)/total * 100), 2));
                    Console.Read();
                }
            }
            catch (ManagementException e)
            {
                Console.WriteLine(value: e.Message);
            }
        }
    }
}