A good introduction:

• It's boring to scale with Ruby on Rails (by DHH)

Optimizing Rails Apps

• Performance Rails <PDF Presentation> (by railsexpress.de guy)

• A Look at Common Performance Problems in Rails

• RailsBench (benchmarker)

• Ruby Performance Validator (automatic profiling of applications as they run)

Load balancers

• Mongrel + Apache mod_proxy_balancer

• Pound

• Balance

• Pen

Cluster Setups

• The Adventures of Scaling (a 4 part behind the scenes expose some guys that tried to scale Rails to serve 2-million pages a day)

• Scale with Rails <PDF Presentation> (by the Joyent guy)

• Scaling Rails with Apache 2.2, mod_proxy_balancer and Mongrel

• Time For A Grown-Up Server: Rails, Mongrel, Apache, Capistrano and You

Hardware

• Sun Fire T1000 (with mega throughput and stupid numbers of cores and virutal processors)