web based chat app

i am planning to integrate chat application on my website.
the idea is to make online user on my website to chat to each other.
it is using PHP5+jQuery, i want it to be as light as possible. I am not sure
about the database to use, but i have 3 options.. file based, sqlite3 or
mysql5.

i am thinking to use sqlite3 because it is fast but afraid it cannot handle
many user
request (i remember reading it is not so good for multiple user). It is also
possible to use
memory based storage /dev/shm (in debian based, i read somewhere - never
tried yet)

another issue is how to make it light.The chat script should refresh
every 200ms  to get the best result or even 1 second still bring quite a
load to the web server.
anyone ever run a web based application?

or any chat based api i can use out there to save server resource?