Your Processor is pretty strong and should be able to any game from high-ultra when paired with a Serious Gaming GPU
GTX 750ti- well for now it must suffice, same cant be said for the future
Assassins creed unity seems to have a terrible PC port as it requires atleast a GTX 680 to just "run" the game
I would recommend to first check out for the real benchmarks as soon as it releases,
Compare it to your expectations, and then think to purchase
Confidence of running that game may be achieved by buying an ATI Radeon R9 270x 2GB OC
Its almost same price and much better than 750ti
Rest you must have atleast 8gigs of RAM