I'm building a chat system for a company and I'm wondering as to what the best way to build the system would be?

The current setup we have is a Nginx HTTP Server with PHP and Memcacheq (as a message queue that appends the chat messages to the user's own queue). We then poll the Nginx server (through a Comet style request) and query the message queue for updates.

Is it a good idea to use a message queue such as Memcacheq to handle a chat system that has both user-to-user and site-wide chat or is it best to just stick to MySQL?

Thanks!