Well, as most
4E rules questions, the answer is pretty unsatisfying because it's not really defined and finding anything with the horrible index and lack of glossary in the PHB makes using it a pain in the butt that makes you wish for a PDF with search feature.
Anyway. The class of the power is listed on the power line. Whether or not a paragon class is considered to be of the same class as the base class isn't explicitly defined, although the fluffy description of paragon paths implies it pretty heavily.
So for raging the 2 paragon powers at 20 that have the rage keyword may or may not count for Rage Strike's requirement depending on if you consider paragon to be a subset of barbarian or a separate class. However there isn't a class requirement on which power is expended, merely that it is a rage power, so the level 20 powers could be used... In which case I'd round down to the damage a level 19 does.
Or wait for the errat... er update, which may never come.