I had a FX8320, and (still have) the GTX970. GTAV had a pretty huge boost. FPS don't drop anymore, and newer games will also have a pretty decent boost when upgrading from your CPU.
If you think you won't play a lot of CPU intensive games, why not. But I'm really looking forward for the new GPU generation, and if you save for a CPU, maybe even the kaby lake or something will also be released and you'll be on better time.
But if you think you can't live with the 750ti, upgrade, after all, the only time to upgrade is when YOU are unhappy with your system