Memory Speed Question

[Flash19]

In my friend's BIOS it states that the memory speed if 533MHz dual-interleaved. I don't really know what 'dual-interleaved' means and I read somewhere about it doubling the stock speed of the RAM or something. Anyway, does this mean that the speed of the memory is actually1066MHz or 533MHz? Bizarrely, the modules he's got installed are 667MHz so I'm curious as to what the speeds really are.

Thanks.

 

[LinkedKube]

sounds like they're running below spec'd to me,