Usually, upgrading your GPU is great. However, don't push it too far - With that I mean; If you replace your GPU, your CPU might not keep up with the data coming from the GPU.
Also, to be on the safe side, you should upgrade your PSU aswell - Especially since the GTX970 is quite powerful. 600W might be just, just enough. But then there will be very little to no headroom for overclocking or further upgrades.
All-in-all, your build is actually very well balanced. So, if you want to get as far as a GTX970, you'll might have to upgrade both CPU and PSU.
But that's a 'might.'
Your system is very balanced by now. So, my advice; Do a little more research - Then make a conclusion what you should do.
Good Luck!
Axel.