The increased reliance on email in organizations, and the use of email for more than just plain email messages, has ment we are faced with ever increasing Information store sizes. Those organizations running Exchange 2003 Standard Edition will know what I mean, and will be happy to hear that with the introduction of Exchange 2003 SP2 later this year the storage limit on a single information store in Exchange 2003 Standard Edition has been increased from 16Gb to 75Gb. This is great news, especially for those organizations nearly at the 16Gb limit now, or are in the planning stage for a new Exchange system shortly.

The number of stores remains the same for Standard Edition, one Private Information store and one Public Information store.

You can find more information here:
http://blogs.technet.com/exchange/archive/2005/06/06/405933.aspx
http://www.microsoft.com/exchange/downloads/2003/sp2/overview.mspx