I have gone through the recomended system specs for Exchange and thought I configured our system correctly but it is constantly running at 95% physical memory usage or higher. Store.exe is using 15 - 17 GB at all times. I'm wondering if I should just install another 2x 8GB sticks but if store will just use all available memory that won't help. Have a new installation of Exchange 2010 running Transport, CAS, and Mailbox roles. Also have forefront for exchange installed. System has 2 xeon E5520 processors and 24GB of ram. I have the page file set to use ram size +100MB.

Yeah, that is normal. This is due to database cache used to reduce I/O operations on servers running the mailbox role. This article explains it in depth: Understanding the Mailbox Database Cache http://technet.microsoft.com/en-us/library/ee832793.aspx If you add more memory at any point, keep in mind the database cache will also increase. Check under the section "Default mailbox database cache sizes" in that article to get ball park numbers.