If you use MyISAM, you won't be doing any transactions per hour, unless you consider each DML statement to be a transaction (which in any case, won't be durable or atomic in the event of a crash).
Therefore I think you have to use InnoDB.
300 transactions per second sounds like quite a lot. If you absolutely need these transactions to be durable across power failure make sure your I/O subsystem can handle this many writes per second easily. You will need at least a RAID controller with battery backed cache.
If you can take a small durability hit, you could use InnoDB with innodb_flush_log_at_trx_commit set to 0 or 2 (see docs for details), you can improve performance.
There are a number of patches which can increase concurrency from Google and others - these may be of interest if you still can't get enough performance without them.
