Yes you can use any Dolby Digital decoder. A great value for the buck is the Denon AVD-2000. You can get it for around $299 and it sounds awesome and has 5.1 channel inputs so you can upgrade to DTS later on if you want to. Or you could get into the Techniques SHAC-500D. It has both Dolby Digital and DTS, although it is not near as well built and doesn't come close to the sound quality of the Denon. Hope I helped.
RCA5451 DSS System with AC3
I have recently purchased a RCA5451 DSS system that does AC3. I have an AIWA surround system that is "upgradeable" to support AC3 but I am not sure what I need? It has the 5.1 plugs on the back but I guess it is just missing the processor. Is there a specific processor I need or could I use any processor such as a Sony SDP-777ES or Yamaha DSR-70 Pro Digital Surround Processor? I am sure this is a dumb question to you experts but I am a newbie. Thanks in advance! :-)