How long will you need the audit data to remain online so that you can run necessary reports? The length of time, combined with how much audit data you will be accumulating, will allow you to plan for how much physical disk capacity is needed and whether you will require multiple physical databases in which to store the audit data. Any long-term storage database should be tuned to store large amounts of data and be very secure, since auditing can accumulate data at a very fast rate.