depends what you use your PC for I guess...
- for gaming you are probably fine. You did not specify your gaming resolution, although I guess it is 1080p. If you want to upgrade and gaming is your main activity, a GTX670/680/690 will be your best bet. Or if you want to be cheap and don't mind SLI, a second GTX480. Those are really cheap now and will give you great perfs for the buck.
- your Mobo/CPU is OK for gaming, but current intel offerings are much better. Effect will only be seen on really computer intensive tasks, like video encoding or professional software. A i5 3750K costs $200 at microcenter, a good MB about the same. You can even keep your memory.
You already have a great SSD and 3TB of storage, blueray... not much you can add so far, you know...
As the universe is curved, there cannot be a straight answer...