Memory speed and latency hardly effect performance in any app or game. You only need memory that is fast enough to support your FSB.
So if you have a CPU with a 1333 FSB rating, you get 333FSB or DDR2-1333 memory. But if you want to overclock that CPU to 400FSB, you need DDR2-1600.
FSB x 4 = DDR2 rating and CPU 'FSB' for the S775 systems. Not sure about AMD. The Core i7 is x8.
Its about the CPU not the memory. So conversely if the Mobo supports 1333, its supports a CPU that has a default FSB of 1333 like E8400. If you only get MoBo that supports DDR2-800 memory, Then you can only use a CPU such as teh E4600 or the C2D Celerons.
It has been proven time and again. Only benchers need 'fast' memory. Its may help a few seconds in some hour long coding/encoding/ripping.