Physics? You measuring the speed of electrons or something?
RIMM 4200s get rid of some problems that DC solutions have. When you stick channels in parallel, you can end up having some major capacitative problems. This can dramatically increase latency on occasion.

It is also easier to impedance match RIMM4200s compared to PC1066, from what I understand.

Overall, RIMM4200s should oc higher. This goes in agreement to what I've heard that they oc quite easily to RIMM4800 which is a 600MHz FSB.

Here's info on the E7500:

http://www.intel.com/design/chipsets/e7500/

It's a huge board!