We show that ultrabroadband parametric generation and wavelength conversion can be realized in silicon waveguides in the wavelength region near 1550 nm by tailoring their zero-dispersion wavelength and launching pump wave close to this wavelength. We quantify the impact of two-photon absorption, free-carrier generation, and linear losses on the process of parametric generation and show that it is difficult to realize a net signal gain and transparent wavelength conversion with a continuous-wave pump. By investigating the transient dynamics of the four-wave mixing process initiated with a pulsed pump, we show that the instantaneous nature of electronic response enables highly efficient parametric amplification and wavelength conversion for pump pulses as wide as 1 ns. We also discuss the dual-pump configuration and show that its use permits multiband operation with uniform efficiency over a broad spectral region extending over 300 nm.