  We discuss the PPN Solar-System constraints and the GW stochastic background
considering some recently proposed $f(R)$ gravity models which satisfy both
cosmological and stability conditions. Using the definition of PPN-parameters
$\gamma$ and $\beta$ in terms of $f(R)$-models and the definition of scalar
GWs, we compare and discuss if it is possible to search for parameter ranges of
$f(R)$-models working at Solar System and GW stochastic background scale.
