Scaling postgres

Hello all:

I'm curious about the limits to postgres' scalability. More exactly, I'm
curious exactly how to make it scale. I'm trying to develop specs for an
application which must support a minimum of 800 concurrent users, up to
double that in short peak periods. I'm hoping to do this with a Postgres
back end and an Apache/PHP front end.

On the front end, as I understand it (I know this is not really
Postgres-specific), with Apache and mod_php I need one process per
concurrent user. Anyone care to speculate how many concurrent users I could
get on a single box? I really don't know what's reasonable to expect.

On the back end, is there any direct relationship between the number of open
client connections and the number of processes used? (I guess that's another
way of asking whether client connections are handled in a multithreaded
fashion). My question again would be, is it at all reasonable to think that
the postgres back end, running on a single box, could handle 800-1200
concurrent users? Is it a matter of running multiple postmasters?

If I can't get all those users on one back-end box, how do I distribute them
across multiple servers but have them all access the same data store?

Thanks,

steve