What would you recommend for a high traffic ajax intensive website?
On the DB question, I'd say PostgreSQL scales better and has better data integrity than MySQL. For a small site MySQL might be faster, but from what I've heard it slows significantly as the size of the database grows. (Note: I've never used MySQL for a large database, so you should probably get a second opinion about its scalability.) But PostgreSQL definitely scales well, and would be a good choice for a high traffic site.
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?