We are in the process of creating a "Design Center" to house
some top notch PC's with Pro/E Flex 3C License Packs loaded.
What is the fastest pc config we can use while keeping XP?
Look at the thread about benchmarks. Then look at the results pages to see what is the fastest. None of them cover the FEA side of things. Those can be set up to use multiple processors. So that is a legitimate option for you.