I know, I know, the P word doesn't give us warm and fuzzies but you have to do it.  Enough said.

If you are in the planning process, I encourage you to check out the large number of resources available.  I've been going through this process pretty heavily lately so here are some useful links.  The first link is a newly published white paper with also good references.

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White paper: Performance recommendations for storage planning and monitoring

http://technet2.microsoft.com/Office/en-us/library/ca472046-7d4a-4f17-92b1-c88a743a5e3c1033.mspx

 

About performance and capacity planning (Office SharePoint Server)

http://technet2.microsoft.com/Office/en-us/library/031b0634-bf99-4c23-8ebf-9d58b6a8e6ce1033.mspx

 

Estimate performance and capacity requirements for Internet environments (Office SharePoint Server)

http://technet2.microsoft.com/Office/en-us/library/45997bad-da15-4442-8727-78e0f2dc5a9c1033.mspx

 

Estimate performance and capacity requirements for Windows SharePoint Services collaboration environments (Office SharePoint Server)

http://technet2.microsoft.com/Office/en-us/library/0a7b2b45-f633-46d2-a4fd-78691d4b8f631033.mspx

 

SharePoint Capacity Planning Tool

http://technet.microsoft.com/en-us/library/bb961988.aspx

 

SharePoint Disk Allocation and Disk I/O

http://blogs.msdn.com/joelo/archive/2007/09/12/sharepoint-disk-allocation-and-disk-i-o.aspx

 

Capacity Planning Key Info

http://blogs.msdn.com/joelo/archive/2007/07/09/capacity-planning-key-links-and-info.aspx