If you were saying that your PSU has the ATX 20+4pin then every PSU has that plug nowadays; it's the PCI-E 6/6+2 pins plug that you should be looking for on your PSU, and yes this mean I'm telling you to unplug your PC from the power socket, open your case, then using your eyes to check if your PSU has that plug or not; while you're at it, note the number of Amperes the PSU has on its 12V rail. For the GTX750Ti the 12V rail should have at least 20amps on it.