memory upgrade

It really matters little which slots you use when adding memory.

But here's Apple's guidance:

Install memory in an iMac - Apple Support

To make it simple, when you add the new DIMMs please install them in the empty slots. iMacs prefer having RAM installed in matched pairs. As the existing 4GB DIMMs are already installed as matched pairs then you would populate the empty slots with matched pairs too.

Finally, please note that iMacs are sensitive to using quality RAM and most users on these forums only recommend RAM from Crucial.com or Macsales.com. Both offer high quality products at very reasonable prices, offer lifetime warranties on RAM and overall provide great service. In short, there is no reason to not use one or the other.