There are a number of tools and utilites that can be used when deciding to build and tune a Microsoft Exchange Server.

Load Sim – Is a bench marking tool that help get a baseline for your server
http://www.microsoft.com/downloads/details.aspx?FamilyId=92EB2EDC-3433-47CA-A5F8-0483C7DDEA85&displaylang=en

Link for performance tuning Exchange
http://www.microsoft.com/technet/community/events/exchange2003/exc-06.mspx

Exchange Server Techcenter
http://technet.microsoft.com/en-us/library/bb124129.aspx

Optimizing Storage for Exchange 2003
http://technet.microsoft.com/en-us/library/fa839f7d-f876-42c4-a335-338a1eb04d89.aspx

Peformance Counters
Perfmon is a great tool that can be used to monitor an existing Exchange server for bottlenecks. The following counter are commonly used to monitor an Exchange Server
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Memory
Memory/Pages per sec
Memory/Available MB
Process/Working Set

Processor
Processor/% Processor Time

Disk Counters
LogicalDisk/Avg. Disk queue length
LogicalDisk/Split IO/Sec
LogicalDisk/% Disk Time

Network
Network Interface/current bandwidth
Network Interface/Bytes Total/Sec
Network Interface/Packets Outbound Errors
Network Interface/Packets Recieved Errors
Tcpv4 /Segmnets Retransmitted/Sec

Exchange Specific counters
MSExchangeIS/Rpc avg Latency
Database/database cache % hit
MSExchange ADAccess Domain Controller/LDAP Read Time