As you probably know, the Standard Edition of Microsoft Exchange is much more limited than the Enterprise Edition. Besides only providing for one information store, the Standard Edition limits your Exchange mail storage—to 75GB with SP2 and to 16GB without SP2. While at first glance a single information store might not seem like much of a limitation, bear in mind that there are a number of global properties for an information store, such as the mail quota, that are easier to manage when you provide multiple stores for your data.
The good news is that Microsoft now makes "step up" licenses available to allow you to move from the Standard Edition to the Enterprise Edition if you're under volume licensing arrangements. I won't try to give you advice on Microsoft's convoluted licensing schemes, though. Contact your software reseller to find out if you're eligible.
Before you do anything, make sure you have a good backup. Even though the procedure is easy, you never know what might happen.
To upgrade to the Enterprise Edition or Exchange, insert an Enterprise Edition CD into your Exchange server and execute the setup.exe program in the \SETUP\I386 folder. On the "Component Selection" screen, choose the Reinstall option.
You may get errors warning you that newer files will be overwritten. This will happen if you've applied a service pack to your Exchange server. Be sure to reapply the service pack to your server after you upgrade to the Enterprise Edition. Obviously, this process is best done during off-hours.
After the upgrade, go to System Manager, and choose Servers, you will see "Enterprise" indicated under the Edition heading.