One of the problems that is often faced with your scenario is that the TV may or may not have the ability to set the optical input to mix down Dolby or DTS 5-channel audio to L/R 2-channel stereo. I have a Panasonic plasma, and that is the case with my particular model (TH-50PZ85U).
If you don't connect the optical output to an AV receiver or pre-processor that has DACs that can decode multi-channel sound, then you will need a separate DAC that can, if you intend to hook it up to an analog stereo amplifier. Almost all of the separate DACs that are talked about on this site are 2-channel stereo DACs that cannot process a multi-channel digital signal.
There are a few < $100 units that can, but they are far and few between. One of them is the
Gefen GTV-DD-2-AA Dolby Digital Audio Decoder that can accept a Dolby 5.1 optical or coaxial signal and mix it down to a pair of L/R analog stereo RCA outputs.
Check your TV manual or setup menu. If you find that you can set your particular model for a 2-channel stereo audio output, then any number of regular 2-channel stereo DACs will work for you, some as low as ~ $30.
Steve