The DD-5.1 is only practical if you plan on hooking it up to a console gaming system or your home DVD player. I'm not sure if the Extigy does DTS but all Audigy 2 cards do. It would seem reasonable to assume the Extigy 2 would support DTS also.
Basically you can hook it up with your digital output. There's a coaxial(RCA) and an optical line-in for digital. There's also 3 sets of analog lines for your cable/satellite, gaming system, VCR, etc. and a G9 port(Digital DIN). It also has bass management (for large speakers), a programmable 5-band equalizer, Dynamic Range Compression (which most newer DVD players have anyways), speaker delay, environment reverb settings and ProLogic.
I have seen a few on ebay but I hate buying used electronics so I just spent some of my tax return on one directly from klipsch.com.