Alright, seeing as you're new to this, let me explain RAM. The "DDR" in memory means "Double Data Rate." This means that for every clock cycle that the memory goes through, there is an instruction sent on the rise as well as the fall of the clock cycle. This means that for every actual
clock cycle, there are 2 effective
clock cycles. That's what the DDR2-XXX speed denotes. DDR2-533 is memory that is actually
at 266Mhz, but is effectively
533Mhz. Same with DDR2-800; Actually 400Mhz, effectively 800Mhz.
You memory is supposed to run at a 1:1 ratio with your FSB, which on Core 2 Duos is 266Mhz. For this reason, DDR2-533 memory is the memory needed to run a Core 2 Duo system without any overclocking at all. However, some motherboards will take higher-clocked memory and automatically set a ratio that runs the memory at the higher speed (DDR2-667, DDR2-800, DDR2-1066) even if the FSB is at 266Mhz. You can change this setting, and you will have to change it if you want to overclock.
Read this guide
for a basic review of Overclocking.