Depends if you feel like your games need more CPU oomph or would you like to turn up graphics setttings more
I would persoanlly upgrade the GPU when the 7xx and 8xxx launch and then CPU when steamroller[amd potential to match intel] comes out at the end of the year so save up until then Your build is pretty blaanced as it is (cpu being the weaker of the two)(6870 should do well for 1080p medium in most games)
Your stroage is good sicne you got good SSD and tons of HDD's
I would personally go with the GPU, if gaming is your main focus. You will need to have a quality PSU to drive the better (more power hungry) GPU, but if you already have a quality PSU than you should be okay.