It is very hard to provide some sort of equation for server utilization based on rough numbers. As it stands now, 1 user could DOS a server if they wanted to, while the same server could handle 10s of thousands of users if they ‘behave’. I suspect that a beefy machine would have no issues handling what you describe, but you should simulate your usage pattern in a developmental setup to see if any issues shake out.
There are no posted numbers for such a senerio. That said how is several hundred users chatting in MUC any more intensive that several hundred users chatting individually. If anything individual threads would be more intensive because they are going to require separate resources for each thread.