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We investigate the effects of performing channel estimation on an OFDM system in AWGN and Rayleigh channels using non-ideal interpolating and decimating filters. Pilots are used to estimate the combined response of the channel and the filters. By making use of the standard Gaussian approximation to the interference for the AWGN channel, we illustrate how to estimate the symbol error rate of an M-QAM system. By using an improved Gaussian approximation for the Rayleigh fading channel for error rate estimation, we show that this is equivalent to making the assumption that the interference is Gaussian noise experienced in a Rayleigh fading channel. It is shown that the predicted symbol error rate is close to that produced by simulations for a 64QAM modulated OFDM system based on the wireless LAN system described in IEEE 802.11a.