That said, if an amp or DVD player says Dolby Digital (or Dolby Digital 5.1) it will be able to deliver up to and including Dolby Digital 5.1.
The only difference in types of Dolby Digital hardware is whether it can deliver Dolby Digital-EX or not. If something is capable of that (6.1) then it will say so, if it doesn't then it can't.
Also some amps can matrix the Dolby digital EX 6th channel but dont have the licence so dont show the EX logo even though they can do 6.1 (eg Marantz 5200)
but if a disc is only encoded with Dolby digital <less than 5.1> it doesnt matter what the hardware is capable of, the disc is basically the limiting factor. (unless you engage additional processing)
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.