I have an iMac that is several years old..(4 or 5?). It handles the D800 files OK, but is slower with them than with my D7000. It only has 4GB RAM, and is not upgradable. Where I run into problems occasionally is when I transport one of these large files into a NIK program, or one of my other photo-enhancing programs like Synthetik. I do work for a couple of fine art places. I have gotten a "File too large" message a few times.
A new iMac is coming out probably in Dec., a 27 inch one, and it can carry as much as 32GB RAM. I'll probably get 16GB. With Thunderbolt and a faster processor, 1 T hard drive, I should be good for awhile.
It's funny how when I buy a new computer, it is more than adequate to handle my software and camera file demands. Then a few years later, it's not. Latest editions of software keep eating up more memory and hard drive. I could limp along with what I have, but I don't much like limping. I like running.