The GTX 750 Ti would be a perfect upgrade to your system. It doesn't require any additional power from your PSU (it gets all of it's power from the PCI-e slot = 75 watts max).
You should see quite a bit of FPS upgrade going from the GT 120 to the GTX 750 Ti!
The PCIe 2.0 downgraded slot from the PCIe 3.0 GPU shouldn't matter in your case. I think the dual GPU's R9 295x and Nvidia's GTX 790 are the only GPU's that would see some decreased FPS from a PCI-e 2.0, IIRC (read it recently somewhere).