High-Availability Question

I hava a db server (mysql) in my web application, it include 550
tables and about 100000 rows in each table. Now, I want to change the
db server to postgresql and construct a HA environment.

I have two 2U servers to build postgresql server (one is master, the
other is slave), and two 1U servers for any purpose about the ha
environment. And I expect to use the FreeBSD as the operation system.

My request is, when a server fail (no matter the master or slave), the
web server can still access (read/write) the database correctly.

I have three ideas about the HA environment, does anyone give me some advices?
Or guide me to learn more advanced ideas. Thanks. :)

First idea:
I install the pgpool on each web server (the web server farm include
16 web servers), and configure the pgpool as replication mode. The web
application (written by php) access to the db server through the local
pgpool daemon.
The idea sounds a little unusual, dose it seems workable?
I just use very simple sql statment in the web application, no any
complex statment.

Second idea:
I install the pgpool and linux-ha on the two 1U server, and configure
the pgpool as replication mode. The web application access to the db
server through the pgpool daemon.

Third idea:
I install the Slony-I, linux-ha and postgresql on the same server of
the two 2U servers. The web application access to the db server
directly and without pgpool.

sincerely,
proace