You should at least use the much faster MX67.
It is also not just a matter of the amount of CPU punch but the amount of RAM required to do the client tracking.
I imagine the lost productivity with 200 clients could be quite large. I wouldn't personally want to put one of my customers into a situation where it intermittently did not work well.
Go for the MX84, and you wont have any problems - and the users will have a great experience all of the time.