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.
White paper: Performance recommendations for storage planning and monitoring
About performance and capacity planning (Office SharePoint Server)
Estimate performance and capacity requirements for Internet environments (Office SharePoint Server)
Estimate performance and capacity requirements for Windows SharePoint Services collaboration environments (Office SharePoint Server)
SharePoint Capacity Planning Tool
SharePoint Disk Allocation and Disk I/O
Capacity Planning Key Info