Ultimately, if you intend to use MyISAM, you must keep in mind that it
eliminates some of your options. One problem is that MyISAM is very
slow to repair after a crash. Remember, if a crash can happen, it
eventually will, it's just a question of when. And MyISAM doesn't have
recovery -- it only has "repair," which will not necessarily recover
all of your data.
If you are not aware of Percona XtraDB Cluster, it might be
interesting for you. (I work for Percona.) There is also Continuent
Tungsten to consider.
Frankly, though, I'd step back a bit from such microscopic focus on
technologies. It looks like you need advice from someone who's done
this before, to get the high-level things right before you dive deeply
into details. If it's really this important, I personally wouldn't
trust it to a mailing list, I'd hire someone. It's well worth it.
There's Percona again, of course, but there's also MySQL, SkySQL,
PalominoDB, and lots more to choose from.