I'd say - run an 8Gb stick and you'll have enough for all your needs - for years to come that is!!!
I use roughly the same services including plex and I never see it above 8% usage out of the 8gb I have installed.
Btw, FreeNAS sucks bigtime unless you run it on UPS and with ECC ram. If not, you will be sure to loose all your data at some point due to improper SD or Ram error. Trust me - been there.. done that.
FreeNAS is perfect in the perfect environment, but for the most of us, the perfect environment is too expensive.