To have a rule in the heading just uncomment the line
\makeheadrule{standard}{\textwidth}{\normalrulethickness}
To have a chapter name typset in a bigest size font replace \Large with \LARGE, \huge or \Huge in
\renewcommand{\chapnamefont}{\centering\normalfont\Large}
Best Answer
Actually Section 7.3 of the memoir manual is very clear in this point.