Virtualization vs. sharing a server

At this moment we are considering a plan for upgrading many servers in our
datacenter.
As this is a very heterogeneous platform (from Oracle 10g, SQL Server 2008,
Firebird 2.1, Mysql 5, Postgre 8.4 up to Informix 5 and even COBOL apps) whe
are evaluating Virtualization or just sharing a server among PostgreSQL and
firebird or MySQL.
As Virtualiaztion is almost discarded after reading many articles that
locate this option in the "don'ts-list", we want to know your experience (if
any) about installing a PostgreSQL and a Firebird server in the same Linux
server.
Postgre is the DB that we are trying to migrate from Firebird, but this
process will demand several months to conclude.

1. Is there a configuration option we need to consider to share this
server?
2. What is the recomended hardware for a PostgreSQL server that has to
support between 500 and 1000 simultaneous user connections? Is there a
whitepaper about this?

Thanks in advance.

--
Ramiro Barreca
rbarreca(at)gmail(dot)com