Here is a suggestion for your memo. (I have been editing for non-native writers of English for 7 years now.)
To： All departments
From: Accounting Department
Date: April 11, 2012
Subject: Overtime pay
To ensure that all employees are compensated accurately for their overtime, the Accounting Department needs the names of the employees who worked overtime during March and the hours and dates that they worked that overtime. Please have your department manager's assistant e-mail this information as an attachment to the Accounting Department before April 13th. We will calculate their overtime pay and distribute it to their salary cards before April 20th. Please submit the file as soon as possible. Thank you!
(Certainly, there are many other options, and I'm not too fond of my repetition of overtime in the second sentence but it is clear and correct). Good luck with your memo! :)