Dell XPS 8700 Desktop Memory Upgrade.

I've just bought a Dell XPS 8700 Desktop (arrived today). Memory wise it currently has 8GB Dual Channel DDR3 1600MHz - 2 DIMMS.

I wanted to upgrade this to 16GB and before ordering the PC I spoke to a Dell Sales representative and he informed me to do this I could buy an additional 8GB memory module and install this myself. I followed his advice and went ahead and bought the extra 8GB memory module.

When I came to install the memory I looked for some instructions on the internet on how to do this and whilst looking at this I found a worrying piece of information in that for the best performance they recommend using matched pairs of modules. Looking at my Desktop spec it has 8GB using 2 DIMMS, I persume this means it's using 2x4GB memory modules. 

Does this mean the Dell sales representative got his advice wrong and instead of recommending I buy the 1x8GB memory module I should have bought 2x4GB?