[Rick Lang] “Dwain, if you are not constrained by budget, I’d recommend you max out the iMac to extend its useful life by a few years for your video use. In other words, buy the largest GPU you can, and select the most internal flash storage you can (the whole machine will run faster with 1 TB of internal flash storage, faster than even 512 GB of flash).”
I definitely second Rick’s suggestion here. Back in late 2013, I purchased a 15″ MacBook Pro Retina display and maxed it completely out (i7 2.6GHz, 16GB RAM, 1TB of SSD space, the 750M with 2GB). Now Apple has refreshed it and yet mine is still quite viable as the new one has the same processor family and AMD GPU tech from 2012 (remains to be seen if it performs significantly over the 750M in FCPX).