What would you recommend for a high traffic ajax intensive website?
I can't speak to the MySQL/PostgreSQL question as I have limited experience with Postgres, but my Masters research project was about high-performance websites with CherryPy, and I don't think you'll be disappointed if you use CherryPy for your site.  It can easily scale to thousands of simultaneous users on commodity hardware.
Of course, the same could be said for PHP, and I don't know of any reasonable benchmarks comparing PHP and CherryPy performance.  But if you were wondering whether CherryPy can handle a high-traffic site with a huge number of requests per second, the answer is definitely yes.
For a website like reddit with lots of up/down votes and lots of comments per topic what should I go with?
Lighttpd/Php or Lighttpd/CherryPy/Genshi/SQLAlchemy?
and for database what would scale better / be fastest MySQL ( 4.1 or 5 ? ) or PostgreSQL?