I’d buy as much RAM as you can afford. If you’re buying the 21.5″ iMac remember that the RAM is now soldered onto the motherboard. So whatever you buy is what you’re stuck with for the life of your machine. If you buy the 27″ iMac then you still have access to the RAM and can upgrade at a latter date as RAM becomes more affordable.
https://www.macrumors.com/2012/10/23/21-5-imac-has-no-user-upgradable-ram-27-has-four-accessible-ram-slots/
Because FCPX and Mountain Lion (10.8) are 64-bit respectively they can take advantage of all the RAM you can stuff into a iMac. If you have more specific FCPX questions I would direct you to the appropriate forum:https://forums.creativecow.net/fcpxtechnique
This forum is for legacy FCP (FCP 1-7).
Ryan Holmes
http://www.ryanholmes.me
vimeo.com/ryanholmes