Performance counters and thresholds for Exchange 2007
Common counters for all server roles
https://technet.microsoft.com/en-us/exchange/cc671175(EXCHG.80).aspx
Client Access Server
https://technet.microsoft.com/en-us/exchange/bb201674(EXCHG.80).aspx
Unified messaging
https://technet.microsoft.com/en-us/exchange/bb201671(EXCHG.80).aspx
Hub Transport Server
https://technet.microsoft.com/en-us/exchange/bb201704(EXCHG.80).aspx
Mailbox server
https://technet.microsoft.com/en-us/exchange/bb201689(EXCHG.80).aspx
Other useful white papers:
Continuous Replication Deep Dive
https://technet.microsoft.com/en-us/library/cc535020(EXCHG.80).aspx
Planning for Large Mailboxes with Exchange 2007