How much bandwidth usage should I allow for if I had a chat going with X number of people.

Bandwidth is usually very little with chats. One way of calculating usage is:

If one Person sends a message of about 50 characters every 20 seconds or so. This results in the same 50 characters being propagated to everyone else in the chatroom:

From the Web Crossing Server Perspective:

Total Chars/20 seconds	People In Room	Message from Each User	Received	Sent KB/s
1	50	50	50	0.0024 KB/s
2	100	100	200	0.0098 KB/s
10	500	500	5,000	0.244 KB/s
100	5,000	5,000	500,000	24.4 KB/s
1000	50,000	50,000	50,000,000	2441 KB/s (2.4 MB/s)

So for the high traffic case of 1000 simultaneous users, each sending a 50 character message every 20 seconds, it would take 2.4 Megabytes per second of bandwidth.