With the E6750, I would suggest getting at least the DDR3 1333 to match the processor's FSB.
No, the RAM is bottlenecked when it surpasses the bandwidth of the FSB.I second that. You always want to try to match your processor's FSB with RAM speed if you can.
, unless you overclock and get the memory to 400mhz (ddr2/ddr3 800). Is that about right?1333MHz FSB = 333MHz = DDR 667