FileInfo[] files = dir.GetFiles();

Array.Sort(files,
                    delegate(FileInfo f1, FileInfo f2)
                    {
                        return f1.Name.CompareTo(f2.Name);
                    }
                );

List<char> levelChars = new List<char>();
levelChars.AddRange("+%@&~".ToCharArray());
List<string> names = new List<string>();
names.AddRange(new[]{"~user1", "~user84",  "@user3", "&user8", "+user39", "user002", "user2838", "%user29"});
names.Sort((x,y) =>
               {
                   int xLevel = levelChars.IndexOf(x[0]);
                   int yLevel = levelChars.IndexOf(y[0]);

                   if (xLevel != yLevel)
                   {
                       // if xLevel is higher; x should come before y
                       return xLevel > yLevel ? -1 : 1;
                   }

                   // x and y have the same level; regular string comparison
                   // will do the job
                   return x.CompareTo(y);                       
               });