Bill Gehrke and Harm Millaard have put together a performance test suite for Premiere Pro CS5 and CS5.5, PPBM5.
You can run the suite of tests and use the results to determine where the performance bottlenecks are with your computer system. The results posted on the website give really good insight into what factors matter for performance, even without you running the tests yourself.
There’s additional information and links to other resources about setting up a system to optimize performance here:
“optimizing for performance: Adobe Premiere Pro and After Effects”