I upgraded my old iMac to the new 27 inch one, with 16GB RAM and 3TB HD. This thing is blazing fast, and handles my D800 files quickly. Amazing. I could have stuck with the other one if I could have upgraded the 4GB RAM, but it was not upgradable.
My new one can be upgraded to 32GB RAM if ever needed, which I can't image I would ever need to. But then, 4GB RAM did me fine for over 5 years until I bought the D800. Who knows what the future holds? It seems every bit of software gets bigger and more bloated with each new rendition.