Actually, interestingly, it's really not the RAM speed- using 667 MHz vs. 800 MHz (which, btw, even in terms of % isn't a huge increase) has 0% influence on speed according to BareFeats- and the bus speed probably isn't really bringing a huge improvement. It's mostly the modified architecture of the new Xeons. They get the same bandwidth and benchmark results with both 667 and 800 DIMMs installed.. but get higher bandwidth than the previous generation at the same memory speeds.
But yeah. The newer ones are faster.