Yes, TopMost works immediately.  If you want to ensure the other form you
are showing appears above the current form, you can:

    Form form = new Form();
    form.Text = "Test";
    this.TopMost = false;
    form.ShowDialog();
    this.TopMost = true;

Note that if you show the other form with the Show() method instead of
ShowDialog(), then the "this.TopMost = true" line will execute immediately,
and your other form will not appear above your main form.  In this case, the
only way to ensure the other form appears on top is to set its TopMost
property to true.