Speed of the RAM and capacity of the RAM are two different things. Your 64 MB RAM chip has the capacity to hold and process 64MB of data. The speed with which that data is transferred to the other computer components is determined by what's called FSB (front side bus) speed.
There are a couple of rules when it comes to RAM. First of all, you need to have the "type" of RAM that your system needs. Your P4, for example, needs the DDR RAM - not the SD RAM. These should not be mixed. Second, the speed of the RAM should match the speed of your board, which should match the speed of the processor. The speed and type of RAM needed are determined by your motherboard. Look in the manual for this information. Another thing I prefer is to use identical RAM in all the slots. I don’t like to mix RAM from different manufacturers; I like it “identical” – absolutely identical. (I don’t care what anyone else says about mixing them. I don’t do it.)
As far as your friend’s comment about the computer running as slow as the lowest RAM chip – he’s partially correct. The system will run at the speed of the lowest component, regardless of which component. (A 133 MHZ speed RAM chip in a 100 MHz motherboard speed will run at 100MHz.) The bottom line, however, is this: the RAM speed should MATCH the board speed. There is the school of thought that it’s ok to put a 133 MHz RAM chip in a 100 MHz system, because the RAM will only run at the100, but I don’t like this practice. (Again, I don’t care what anyone else says about how using a higher speed component is ok, but using a lower speed is not. I won’t do it. )
