Memory: Single-Rank, Dual-Rank, what?

I was wondering if someone could please clear up some confusion for me. I've been looking into getting a 4x 1 GB FB-Dimm package from vendors such as Ramjet.com, Otherworldcomputing.com, Datamemorysystems.com...etc., and some of these sites list their memory as Dual Rank.

Now, I've read articles and have sent questions to some of these vendors asking if their FB-Dimms are Dual Rank or not and some of the information they have sent me regarding Dual Rank memory seems to be conflicting. and even some review sites such as Anandtech.com that say Dual-Rank memory is preferable and will see performance boosts (Anandtech.com article linked here: http://www.anandtech.com/mac/showdoc.aspx?i=2832).

Here are a couple of replies I got from John Mayer from Ramjet.com after asking whether or not their memory they sell for the Mac Pro is dual rank or not.:

"Brian,
Al of the different FB-DIMMs that we sell for the Mac Pro are single
rank. There are dual rank modules out on the market for a lower
price, but they are not recommended for use in the Mac Pro."

and this one after I copy/pasted him to a passage from the Anandtech article (the portion where Anandtech states Dual Rank if preferable):

"Brian,
I don't know where that text came from but it is incorrect (perhaps
they are confusion dual rank with dual pair). The Mac Pro has 8
slots, and is capable of addressing 8 ranks. Therefore, if you
install 4x dual-rank modules, you will be using up all 8 ranks, and
you will not be able to install any further RAM (i.e. the remaining 4
slots must be left open). This is why Apple specifically recommends
against using dual rank modules.

John"


So which is it? Is the term Dual Rank being used erroneously by some of the vendors and review sites out there or is Dual Rank really what I want contrary to what John from Ramjet.com told me? I'm looking to get the best performance out of my Mac Pro.