I jave intel core 2 duo e7400 2.80ghz
Ram 4gb ddr2
Psu 550 watt
Im planning to buy a gtx 750 ti or gtx 1050 ti which is better of these 2 cards for gaming
the 750ti is about as strong as you want a gpu to be on a core 2 system. jumping to a 1050 will just cause judder and hitching as the gpu is just to powerful in comparison to the cpu.
in this case i would say save your money and buy a new base system (cpu motherboard and ram) then consider a gpu upgrade.
At stock speeds on the cpu, a 750ti is about as good as it gets. If you can bump up the OC on a q6600 to @ 3.2GHz, then I'd say either of the 1050/1050ti would be max.