do i still need an amplifier if my dvd player has a built in dolby digital 5.1 decoder
Yes, to amplify the sound so that it is loud enough to drive your speakers.
To expand on Derek's comment (and he is correct)You will either need a receiver, an integrated amp, or a separate multichannel amp and a/v pre-amp.
The signal output from a tuner, CD, dvd player, or phono player is only sufficient to be processed by the very sensitive pre-amp section of a receiver or pre-amp section of integrated amps or a separate pre-amp with a separate amplifier.. All components need amplification to drive speakers. High fidelity speakers need considerably more power than could ever be gotten from dvd players to move the speakers to push out a significant soundwave.