Let chapter number be distinct in mainmatter and ToC

To redefine the formatting of the chapter entries you can use either the tocloft or the titletoc package; below there's an example with tocloft showing the modifications required to include "Serie" before the chapter numbers and a colon immediately after the chapter numbers:

\documentclass{book}
\usepackage[titles]{tocloft}

\newlength\mylen
\renewcommand\cftchappresnum{\bfseries Serie~}
\settowidth\mylen{\bfseries\cftchappresnum\cftchapaftersnum}
\addtolength\cftchapnumwidth{\mylen}
\renewcommand\cftchapaftersnum{:}
\renewcommand{\thesection}{\textrm{\Roman{section}}}

\usepackage{titlesec}
\titleformat{\chapter}[display]{\filcenter\sffamily} 
{\textit{\LARGE\@title}\\\vspace{5mm}
{\Large\thechapter}}%
{1pc}{\titlerule\vspace{1pc}\Huge}%
[\vspace{1pc}\titlerule\thispagestyle{plain}]

\begin{document}
\tableofcontents

\chapter{Test Chapter One}
\section{Test Section One One}
\section{Test Section One Two}
\chapter{Test Chapter Two}
\section{Test Section Two One}
\section{Test Section Two Two}

\end{document}

After the edit to the original question, here's some code in which I used \addto\captionsfrancais to change \chaptername in the preamble:

\documentclass{book}
\usepackage[francais]{babel}
\usepackage[titles]{tocloft}
\usepackage{titlesec}

\newlength\mylen
\renewcommand\cftchappresnum{\bfseries\chaptername~}
\settowidth\mylen{\bfseries\cftchappresnum\cftchapaftersnum}
\addtolength\cftchapnumwidth{\mylen}
\renewcommand\cftchapaftersnum{:}
\renewcommand{\thesection}{\textrm{\Roman{section}}}

\titleformat{\chapter}[display]{\filcenter\sffamily} 
{\textit{\LARGE\@title}\\\vspace{5mm}
{\Large\chaptername~\thechapter}}%
{1pc}{\titlerule\vspace{1pc}\Huge}%
[\vspace{1pc}\titlerule\thispagestyle{plain}]

\addto\captionsfrancais{\renewcommand{\chaptername}{Serie}}

\begin{document}

\tableofcontents

\chapter{Rappels}
\section{Section}
\begin{figure}[h]
\centering\fbox{A figure}
\caption{A caption} 
\end{figure}

\begin{table}[h]
\centering
\begin{tabular}{ccc}
\hline
aaa & bbb & ccc \\
\hline
\end{tabular}
\caption{A caption} 
\end{table}

\end{document}

The toc setting is defined by \l@chapter which in book is

\newcommand*\l@chapter[2]{%
  \ifnum \c@tocdepth >\m@ne
    \addpenalty{-\@highpenalty}%
    \vskip 1.0em \@plus\p@
    \setlength\@tempdima{1.5em}%
    \begingroup
      \parindent \z@ \rightskip \@pnumwidth
      \parfillskip -\@pnumwidth
      \leavevmode \bfseries
      \advance\leftskip\@tempdima
      \hskip -\leftskip
      #1\nobreak\hfil \nobreak\hb@xt@\@pnumwidth{\hss #2}\par
      \penalty\@highpenalty
    \endgroup
  \fi}

So you can redefine that and add Chapter at a suitable point (just before #1 for example)