precision 690 1kw chassis memory upgrade

Hello!

I have a Dell Precision 690 workstation. It's the version with the 1Kilowatt psu and the riser card which provides 16 memory slots.

At the moment there are installed only 4GB of Ram. 4x1gb DD2 667 Micron Technology FBB ECC DUAL RANK Ram.

When I go to the Kingston Memory Configurator it suggests to buy SINGLE RANKED Memory. Ok, it's logical. The MOBO has 8 slots, so if you want to use all 8 you have to buy single ranked. Otherweise you can use only max 4 slots.

But in my case it`s different: I use the Riser Board, which is attached to only 4 of the 8 slots. 

So is it correct, when I'm saying that with the riser board it makes no difference installing SINGLE RANKED or DUAL RANKED?

I'm sure one of the experts can help me :)

And btw. sorry for my bad english....

Greetings from Milan