but Krishna and Rama are both incarnations of Vishnu, no?
Yes


are the "holy texts" 'holy' in the same way as Bible, ie, taken to be "word of god"? or taken more allegorically/metaphorically?
Depends on who you talk to. To the man on the street, they are holy in the same way as the Bible. To most intellectuals, these are holy texts in the sense that they define the religion.

digging around the net, seems that Rama, Krishna are 'from' Vishnu, but there's also Brahma and Shiva. Are those three from a single god?
Is that what you mean by 'layered' .... that originally there were lots of gods and as the religion evolved, the lots were seen to be 'from' a few and then the few from one?
Brhama, Vishnu, and Shiva are the Indian Trinity -- Gods of Creation, Life, and Death (and hence Life -- remember the Circle of Life!), respectively. Yes, by "layered" I did mean that originally there were lots of Gods (Indra= Lightning = Zeus or Thor; Agni=Fire; etc.) and as the religion evolved, the lots were seen as from one.

and there were later passages i think make it clear that they had the concept... say "billions and billions" and maybe you just mean "lots and lots", say (something like) "a day and a night to Brahma is 4 billion years" and it must mean something more than "4 times lots"
I would have to go read that passage to what it says in Sanskrit, but then since my knowledge of Sanskrit is equivalent to that of Latin for most native-born Americans, I'll probably get it wrong :-)

again, thanks for your thoughts.
You are welcome, jp!

-- Shreeder ("adiabatic")