Right now I have no page outs and hardly ever run lower than 2-3GB to spare. I even tried to shake it running Windows 8 in a VM and gave it all 8GB of memory. I still couldn't shake it. I think this will run FCP X WELL.The upgrade from 2 GB will seem like a massive increase already. 8 GB should be fine, but do you want fine or amazing? You can check how much page outs you have right now. Restart the mac at the beginning of the day and then do your normal work stuff. Then check your page outs in activity monitor at the end of the day.
Note again this is the baseline 21.5" 2012 model.