Scalability - 000s of concurrent users

[wrn]

We need an extremely simple chat app to sit on the homepage of a site which regularly receives many thousands of concurrent visitors.

Only a small percentage of these visitors will be logged in to chat - but we want all non-logged in visitors to be able to view the chat. I assume that this means phpFreeChat would need to maintain a lot of concurrent sessions - how many can it support on an average server? Can the load be balanced across multiple servers if required?

If phpFreeChat cannot support this many users, does anyone have any suggestions?

Help appreciated.