There are people who want the fastest computer and people who want the fastest computer as cheaply as possible.
If you want a fast PC without overclocking, then you have to spend a lot more money, but you would end up with things like PC8500 RAM because you can run a genuine 1:1 with a non-overclocked 266MHz quad-pumped CPU.
CAS5 PC8500 RAM allows more read and write cycles per second than CAS3 PC5300 RAM. That's an unarguable fact. So, as until you get CAS1 RAM, the CPU is always waiting for RAM lookups, the faster RAM will give higher performance in memory intensive benchmarks like SuperPi and some games like BF2142.
Given that you can buy 2Gb of PC8000 Ballistix for £103 delivered, I think you'd need to be daft not to buy performance RAM as it will tolerate more upgrades than PC4200 or PC5300 RAM.