I'm not a database expert, and I do not speak from experience. However:
MyISAM tables use table-level locking. Based on your traffic estimates, you have close to 200 writes per second. With MyISAM, only one of these could be in progress at any time. You have to make sure that your hardware can keep up with these transaction to avoid being overrun, i.e., a single query can take no more than 5ms.
That suggests to me you would need a storage engine which supports row-level locking, i.e., InnoDB.
On the other hand, it should be fairly trivial to write a few simple scripts to simulate the load with each storage engine, then compare the results.
