I recently read a review about performance with differing speed RAM.
The slowest FSB was 800mhz, the fastest was 1333mhz. The fastest beat the slowest by up to 30% in some benchmarks. How that translates into real-time app performance is variable.
Unless you're certain your apps utilize the CPU significantly, having a fast one is mostly a waste of money. The computer will boot, shut down, and load apps quicker, but most of the work for most apps is done in RAM. The CPU gets very low usage most of the time.
I have a couple of older P4's, DDR400, 2.8Ghz. My C2D rig, DDR2-667, is slightly faster at booting, shutting down and loading charts. The performance difference of the C2D does not justify the cost differential in my use.