We already have such a setting, you can add this to user.config:
<add key="NewsletterMaxToSendPerMinute" value="5" />
Since there are 60 minutes in an hour that should send approximately 300 in one hour. Or you may want to set it to 4 if you have other things such as forums that may also be sending email at the same time.
I've just updated the documentation article Sending Bulk Email to mention these settings.
Hope that helps,