Hi, do you have a 4K display
some more info will help, ram, HD's, OS etc
you can edit 4K, may be a tad slow might be worth looking at proxy workflows or down sampling to 1080p
you want a OS drive, media drive, scratch drive (not quiet shore how that works in FCX) make shore you media is on a different drive to the OS & a fast one (WD black 1TB drives are cheep)
keep your files neat
SSD as boot drive is ok but not shore if it makes any real diffidence to speed when rendering video, your on a macpro 3.1 quad so you must have a small budget so dont look at SSD's.
GPU's are worth looking at what do you have now and do you have much of a budget to play with?
if you are using FCX you want ATI.
always keep your drives as clear as possible never let them reach 80% full or they will relay slow down.
this site is worth a look
http://kenstone.net/
(an SSD as boot drive will speed up boot times and app launch times, but at most you boot once a day and open an app once so not much of a saving of total time spent working.
a fast dedicated media drive will help real time playback and keep you working longer/faster)