Overspeed,

With respect to PC6400 vs PC8500, consider this. PC6400 will support an FSB of 1600MHz at stock speeds, PC8500 will support an FSB of 2132MHz at stock speeds. If your operating FSB is less than 1800MHz (900MHz on the Ram) than you should probably us PC6400. Above 1800MHz FSB you should get the PC8500. Those are just general guidelines. There are plenty of forums and articles comparing the OC capabilites of various memory modules on the net.

I personnally chose the PC8500 for several reasons. First the Mercury build is a testbed, not my primary system, so I wanted flexibilty to evaluate chips like the E8400 at higher FSB speeds. Secondly, while everyone discusses overclocking their memory I feel it is equally important to get a good underclock. My PC8500 is rated 5-5-5-15 @1066Mhz, but I run it most of the time at 4-4-4-11 from 700-900MHz.

If you ask everyone on this board what the best memory is you'll get a dozen answers. It really comes down to your needs and price point.