I remember back when I was studying hardware about when using SIMMs and older DIMMs you would need to use more than one in order to fill the memory bank. (i.e. Need four 8-bit SIMMs to fill the bank of a 32-bit 486 CPU).
What I've been trying to figure out is, is this still an issue? RIMM's are 16-bit but you don't need eight of them to fill a bank for a 128-bit P4. (By the way, is a P4 really 128 bits? I read it somewhere but haven't been able to confirm it anywhere else).