> Is there a list somewhere of the supported boards for SID?
I don't know of a single list. The bin/configrun-sid.pl perl script
and the main/dynamid/*.cxx code are the two places where board concept
> Also, is there a decent howto for adding new boards to the sim [...]
The outline is easy: pick all the component models (component type
names), memory map(s) (address ranges for RAM, peripherals), and on
the choice of software-oriented models (debugging, profiling
interfaces). Then, either the perl- or C++-based front-ends should be
easily extended with such data for a new board.