Characteristic impedance behavior with frequency
Does R change a lot with frequency?

\$R\$ tends to increase at high frequency due to the skin effect. This is because high frequency currents penetrate less deeply into the conductors, reducing their effective cross-sectional area. This results in \$R\$ increasing proportionally to \$\sqrt{f}\$.
\$G\$ also tends to increase at high frequency due to the polarizing behavior of the molecules in the dielectric being unable to respond immediately to the applied fields.
In transmission lines designed for high-frequency transmission, increasing \$R\$ tends to become important well before increasing \$G\$ does.