kemp released a cool tool: LoadMaster Sizing Guide for MS Exchange to see which of our products is best for you.
Some info about Kemp:
KEMP loadmaster combines versatility with ease-of-use to speed deployment (see our step-by-step guidelines in the LoadMaster Deployment Guide for Load Balancing MS Exchange 2010) of the complete portfolio of advanced messaging applications and protocols used by exchange 2010, including outlook Web Access, Outlook Anywhere, activesync, SMTP, POP3 and IMAP and rpc Client Access (Native MAPI). With built-in SSL acceleration and/or overlay, the LoadMaster offloads a key source of CPU drain to improve the capacity of Client Access Servers. Layer 7 healthchecking at the LoadMaster ensures that should one of the servers become inaccessible, the load balancer will take that server off-line, while automatically re-routing and reconnecting users to other functioning servers. Not quite ready to make the move to exchange 2010? KEMP LoadMasters integrate seamlessly with earlier versions of Exchange as well. 
Screenshot from de sizing tool:
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