I have a mid-2007 24-inch iMac (2.8GHz Core2 Duo with 4GB RAM and 256MB graphics). Unsurprisingly, it crawls to a halt when I work with the large files from my D800 camera. I'm going to upgrade to the new 27-inch iMac. Photos and video are the only processor intensive tasks I do. How much benefit would I get from the upgrade options? 2.9GHz i5 vs 3.2GHz i5 vs 3.4GHz i7 processor? 16GB vs 32GB RAM? 512MB vs 1GB vs 2GB graphics? 3TB Fusion Drive or 765GB flash and an external hard drive? I plan to keep it at least 3 or 4 years. thanks!