XML index support

Dear friends,

You may know me as I worked a long time ago on pgAdmin I with Dave.

As this is an XML related question and XML is quite new, I am posting
on hackers ML.

We would like to develop a free REST database (real-estate standard)
based on a PostgreSQL schema. This is a free solution for free data
also, released under BSD or GPL license. We are charity.

This is supposed to a "killer application", so we need to drive down
queries to 3ms to 5ms to allow hundreds of simultaneous queries.

Php will probably be part of the package.

What is in your opinion the best way to achive this :
* develop a traditional databe and implement materialized views in PL
OR
* implement the new XML type with some index (GIST) designed for
PostgreSQL. Which one?
OR
* write triggers behind XML tables to pullulate a traditionnal database,
add indexes and query traditionnal database.

Kind regards,
Jean-Michel