What would be the peak voltage value of two combined signals (one noise and one sinusoidal signal)?
Most of the noise that occurs in electronic instruments and in environment is additive noise, so yes, if a noise is of same amplitude as signal, both will add to generate higher voltage signal and yes it can saturate the output level.
Usually, at any instant of time the value of output signal is equal to signal value plus noise value at that instant of time.
But with the use of current op-amps which are compensated with higher input impedance, lower output impedance, rail to rail, higher PSRR, CMRR etc. u can get lower noise generated within amplification device.
The other noise u may find is phase noise where phase of signal fluctuates to create jitter. U can go through noises from here http://en.wikipedia.org/wiki/Noise_%28signal_processing%29