Have a look at the front of any DVD player or amp or the feature list of any such product. The term Dolby Digital or indeed the Dolby Digital badge is used to indicate compliance with all Dolby Digital formats up to and including 5.1.
Which is exactly my point, if hardware simply says Dolby Digital it means Dolby Digital 5.1 compliance. So a simple answer to Harri2000's question as I take it is that buying something that says Dolby Digital will give you the same functionality as buying something that explicitly states Dolby Digital 5.1.
But as you say films etc. are a different matter as they may state Dolby Digital but on closer inspection may turn out to be mono, stereo or whatever.