Well yes & no. For P4s where the multiplier is locked, you can only overclock with the FSB & in general that means also overclocking the memory.
But then again, certain boards can run the Ram with a divider where even if you were to raise the FSB, you can use a divider to make the memory run at a lower speed.
Here's an example :
Let's say you have a P4 with a 200 FSB ( 800 ) & some PC3200 Ram. You run both in Sync so 200 FSB = 200 mhz for the Ram ( 400 ). That's what PC3200 is rated for, so no problems so far.
If you start raising the FSB by 1mhz, the memory will follow & its speed will also go up by 1mhz. So if you get your FSB all the way to 250mhz ( 1000 ), you Ram will be running at 250mhz ( 500 ) putting it way beyond its rated speed ( it probably won't even do a POST ). In this case, you would need PC4000 Ram.
Now, if your board has a 4/5 divider for the memory, you can set the FSB to 250mhz & the divider will keep the memory to 200mhz because ( 250 / 5 ) * 4 = 200mhz. In this case, your PC3200 Ram will work fine.
Another matter is the amount of memory bandwidth. The P4 FSB at 200mhz ( 800 ) is capable of adressing 6.4 GB/s ( in theory ). PC3200 can deliver 3.2 GB/s. If you put them in a Dual Channel configuration, then 3.2 GB/s *2 -> 6.4 GB/s. In other words, it's exactly what the P4 needs. Now if you overclock the FSB to 250 ( 1000 ), the FSB can adress up to 8 GB/s ( once again in theory ) & if you keep the Ram at PC3200 in Dual Channel, it will still deliver 6.4 GB/s, thus you will loose some benefit from running a higer FSB. That's why it's better to keep the FSB in sync with the Ram even if you don't overclock.
