Corsair just published a detailed Application Note on memory performance for the Intel 955X Express Chip Set.
Here is the summary of the White Paper:
"We can draw several useful conclusions from the results of this testing. First, we have discovered that selection of the system memory should be tightly coupled to processor selection. 1000MHz memory looks like an excellent choice if you are planning to overclock a processor with native 800 MHz front side bus; if you do not plan to overclock, 800 MHz memory is a good choice. The 667 MHz memory offers a good compromise solution, providing strong performance and excellent flexibility."
The pdf is posted on Corsair's Tech Notes page and can be seen here: