The impact of imperfect estimates on ultra-wideband (UWB) system performance is investigated when path delays and path amplitudes are jointly estimated. The Cramer-Rao bound (CRB) of the path delay estimates is presented as a function of the signal-to-noise ratio (SNR) and signal bandwidth. The performance of a UWB system employing a Rake receiver and maximum ratio combining is analyzed taking into account estimation errors as predicted by the CRB. Expressions of the bit error rate (BER) are obtained displaying the effect of the number of pilot symbols and the number of multipath components on the overall system performance. Allocations of power resources to pilot symbols are determined to optimize the BER. Results indicate that the impact of path delay estimation errors has to be taken into account, particularly at low SNR. Finally, the estimation errors are taken into account to optimize the signal bandwidth and the number of fingers of the Rake receiver in UWB systems. With limited number of pilot symbols (< 10), the optimal bandwidth is smaller than 1 GHz. For a given bandwidth of 2 GHz, the optimum number of paths to be processed by Rake receivers is approximately 20.