Here are the <a href=”http://technet.microsoft.com/en-us/library/aa996719.aspx”>general system requirements</a>.
As you can tell, the server doesn’t need to be too beefy. For 45 mailboxes, you can get by with a 3000 series Intel processor and as little as 4GB of RAM, but since it is a critical app, I would recommend a 5500 series processor with at least 8 GB of RAM.
Storage is very important to Exchange. I would recommend 15K RPM hard drives only. To optimize your storage performance, plan for 3 partitions: OS, Transation Logs, Database. RAID 1 for the OS and Transaction Logs, and RAID 10 for the database.