We've recently been playing with a Dual CPU Quad Core system. (Two Xeon 5400 Quad Core CPUs).

Talk about power!! It was needed for a large ecomerce site that we've been dealing with to allow faster database indexing and searching. We're also running RAID 50 on the machine and 32GB memory. Need power? It's got it! Not cheap, but when the client says "i need" rather than "i want" - you do what's necessary.