Scaling Database for heavy load

Hello,

we use PostgreSql 9.x in conjunction with BIND/DNS for some Companies with about 1.000 queries per second.

Now we have to scale the system up to 100.000 queries per second (about).

Bind/DNS is very light and i think can not give us bottleneck.

The question is how to dimension the backend database.

The queries are select (only few insert or update), but the 100.000 queries per second are only select.

How can i calculate/dimensionate?

We think to put mor ethan one Bind Server (with backend database) behinbd a router with balancing capabilities.

The problem is to know which requirements and limits does a Postgresql 9.x installation - 64 bit - can have.

Furthermore, we tried Rubyrep (it is quite old!); can you suggest me other replication modules that can work also if connction link, from Database Server, went down?

Thank you!

Francesco