PostgreSQL clustering VS MySQL clustering

Dear community,

My company, which I actually represent, is a fervent user of PostgreSQL.
We used to make all our applications using PostgreSQL for more than 5 years.
We usually do classical client/server applications under Linux, and Web
interface (php, perl, C/C++). We used to manage also public web services with
10/15 millions records and up to 8 millions pages view by month.

Now we are in front of a new need, but we do not find any good solution with
PostgreSQL.
We need to make a sort of directory of millions of data growing about 4/8
millions per month, and to be able to be used by many users from the web. In
order to do this, our solution need to be able to run perfectly with many
insert and many select access (done before each insert, and done by web site
visitors). We will also need to make a search engine for the millions of data
(140/150 millions records at the immediate beginning) ... No it's not google,
but the kind of volume of data stored in the main table is similar.

Then ... we have made some tests, with the actual servers we have here, like a
Bi-Pro Xeon 2.8 Ghz, with 4 Gb of RAM and the result of the cumulative
inserts, and select access is slowing down the service really quickly ...
(Load average is going up to 10 really quickly on the database).

We were at this moment thinking about a Cluster solution ... We saw on the
Internet many solution talking about Cluster solution using MySQL ... but
nothing about PostgreSQL ... the idea is to use several servers to make a
sort of big virtual server using the disk space of each server as one, and
having the ability to use the CPU and RAM of each servers in order to
maintain good service performance ...one can imagin it is like a GFS but
dedicated to postgreSQL...

Is there any solution with PostgreSQL matching these needs ... ?
Do we have to backport our development to MySQL for this kind of problem ?
Is there any other solution than a Cluster for our problem ?

Looking for your reply,

Regards,
--
Hervé