I picked up this OCZ 4gb set
earlier in the week (which facilitated my move to Vista64). Although the set is marketed as a 800mhz kit the SPD is set for 667mhz (5-5-5-15). To get the memory to run at 800mhz you need to manually change the memory timings in the BIOS. I haven't made this change yet so I'm still running at 667mhz. My experience index is rating the memory at 5.5. I'm using an Intel e6600 on an Intel 975xbx2 motherboard. Memory is set for dual channel operation.
I don't recall for sure but I think my previous 2gig 800mhz Corsair kit with 5-5-5-15 timings was rated by Vista at 5.6. I'll let you know if my current 5.5 rating changes after I make the change in the BIOS. I'm not planning on making this change for at least a week or two. I want to make sure I've achieved a baseline stability before making any more changes to my setup.