Collaboration as in utilising the public folder system? or a 3rd party product? At this stage (depending on your doing a baseline of exchange servers performance) i'd expect you not to be having many cpu/memory issues.
The problem you are more likely to have is disaster recovery and management of 400-500 users. How long will it take you to get all the data back online if the machine crashes AND you have to recover the collaboration data as well?
It may be time to consider splitting different functions to different machines. Say maybe a dedicated collaboration services machine.
consider it.. otherwise do that baseline of current system performance and then implement your collaborative data you'll see the difference immeadiately depending on utilisation, but i'd expect server to soon require at least a cpu upgrade.
regards,
jb