cpu overclocking riases FSB speed to raise Cpu speed. Ram also runs on the fsb bus, so if you have a 3ghz cpu, and yor fsb speed stock si 200mhz, it would be like this
Cpu is 200mhz X 15(multiplier)
Ram is 200mhz X 2 (DDR means double data rate)[this is 400mhz otherwise known as pc3200)
now say you oc to 3.750ghz
Cpu is 300mhz X 15
Ram is 300mhz X 2 or a hell of a lot higher than 400mhz stock speed.
at tihs point your cpu may be able to get htat high in speed, but yuor ram wont and thus, your ram will limit your oc.
yuo have 2 choices, use a divider, or buy ocing ram.
using adivider looks like this
cpu=300 X 15=3750mhz
ram = 300 X 2/3 X15=400mhz
what a divider does is that it limits how fast a system can access the ram so it runs async from the cpu. in this case, for every 3 million hertz the processor works, the rma works for only 2 million hertz thus effectively slowinng down the speed.
the problem is that it decreases performance. i dont know exactly how much it affects an Pentium 4 but i know it has a huge performance penalty for an Amd Athalon XP and almost no penalty for an AMD Athalon64.