Maxwell architecture is as good as any other for gaming.
When you sort out the gaming benchmarks, the GTX750ti is in the same performance class as the R9-265 and GTX570. One good reason for Maxwell is the lowered psu requirements.
I do not know your currency, but for a balanced gaming pc, I would divide my combined cpu and gpu budget into two parts. The graphics card is of most importance, so I would allocate 2 parts to the graphics card and one to the cpu.
A 4670K is as good as it is going to get for gaming. It will allow you to run a GTX780ti .
If you are building today, buy a 620w psu even if you don't need that much. It will allow you a future graphics upgrade. I suspect you would find a GTX770 to be appropriate, but we all have a budget.
A GTX750ti would not be wrong, just so long as you can be prepared for a future graphics upgrade.
And... ignore mantle as a selection criteria. It reduces the driver cpu load which is irrelevant with a strong cpu like a 4670K.