WHAT IS THE DIFFERENCE IN MACBOOK 4.1. IT STATES PROCESSOR SPEED OF 2.1GHZ WITH A BUS SPEED OF 800 MGHZ AND THE MEMORY SPEED IS 667MHZ
SHOULD I BUY THE MEMORY UPGRADE AT 667 OR 800 MHZ. I WANT TO UPGRADE FROM 1GB TO 4-6GB.
Memory will typically 'underclock'. The hitch could be in the SPD register (on the memory module) and how the motherboard handles it (and manual settings if available). The SPD register contains the suggested timings at different speeds. Most modules contain timings for common speeds, so 800 models should be fine at 667. If timings aren't there, its usually possible to manually set them (at least on PC motherboards).
Like popatim says, most systems seem to be most efficient when the memory and system bus are 'in harmony'