For a typical Exchange 2007 server installation with the roles of Mailbox, Client Access, and Hub Transport the "sweet" spot of memory is around 12GB to 16GB - that seems like a lot because it is! Although an Exchange 2007 server will run with less memory it needs this much to avoid page swapping with as little as ten mailboxes.
Of course if you have more memory installed in the server it will run better, because the extra memory will be used for disk caching. You can compensate somewhat for a slow disk subsystem by installing more memory on the Exchange 2007 server. The additional memory is at least an option compared to Exchange 2003. With large Exchange 2003 installations the ultimate performance of your mail system was determined by the speed of your disk subsystem.
If you're experiencing performance problems on your Exchange 2007 Server and you have less than 12GB of memory consider adding more memory to increase your Exchange Server's performance. To see if your Exchange 2007 Server is memory "challenged," start the Task Manager and click on the Performance Tab. If the amount of free memory is less than 1000MB (1GB) then your Exchange 2007 Server could probably benefit from the additional memory.