Which memory configuration with quad core

Which is the best memory configuration to use with a Q6600 quad core processor ?

a) 2Gb PC-8500 1066 MHz 5-5-5-15 or

b) 4GB PC-6400 800 MHz 4-4-4-12

I'm putting a new PC together and either of these 2 options costs about the same, so would like advice on the best performance option - I intend to overclock the Q6600, but not massively.

(the computer will be used for CAD, Structural Dynamics calcs, Finite Element Analysis and 3D modelling, so number crunching ability is top priority)

TIA

I'd go for b - it's lower latency and there's more of it. I'm guessing the higher FSB won't have as much impact.

Given that you'll struggle to clock a quad core above a 400 MHz FSB, there's no point getting RAM beyond PC6400.

Cheers for the replies, 4GB of PC2-6400 it is then!