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?