Most transactions per second on largest box?

This is a very general question but what is the largest linux box anyone
has run PostgreSQL on and what kind of concurrent transactions per
second have you seen?

We have a client who has huge bursts of activity, coinciding with high
rated TV appearances, meaning hundreds of thousands of users registering
and logging in at the same time.

Currently we are running a dual cpu dell blade server on redhat linux
(2.4?) and PostgreSQL 7.3.4 on i686-pc-linux-gnu, compiled by GCC 2.96,
raid5 and am using sqlrelay for connection pooling. It works fine under
ordinary load but bogs down too much under these huge loads.

I would love to be able to tell them that a specific box like a 2 ghz
quad xenon with 10 GB of ram, on a raid 10 server will get them x
concurrent transactions per second with x threads running. From that I
can give them a great estimate on how many registrations per minute
there money can buy, but right now all I can tell them is that if you
spend more money you will get more tps.

Thanks for any info you may have.