The most significant problem with Playing US DVD's in Europe is REGION CODING. Music DVD's in general though are 'region free' (ie: Set to Region 0 = All Regions). It is also possible to pick up DVD players that play multiple regions if you can't find a DVD in the particular region you want).
Also for many European, or rather I should say PAL countries, it will be found that TV's purchased within a country while designed to play PAL, are ALSO compatible with NTSC televisions formats. Unfortunately it doesn't seem to work the other way around, which is to say that it's slightly harder to get PAL compatible TV's in the US.
So for THIS particular case Korlich, as we already know the Opeth DVD will be region 0 (Free) you're only conern is the playback type of the US DVD's. Though it has already been all but confirmed that at least the first batch will be PAL (Not a problem for you) if they ARE NTSC, now would be a good time to look at the manual that came with your TV, there 's a good chance that your TV is capable of NTSC playback, and the manual should tell you.