Desktop memory differences

I want to upgrape my home pc but, I am not positive which availible types will work on my pc. I have a hybrid pc with a Western Digital HD & Intel processor 300 mhz. My question is which is best to use PC100 or PC133 and 8ns or 168 pin memory. I know that the first category is the speed of the memory, and the 2nd I am not sure. Thank you, Rob