Where do you see a 750ti that requires a 600w PSU?
You will be totally fine with a 400w PSU (depending on it's quality and the rest of your system).
The 750ti can run on a 300w PSU and only requires a measly 20amps on the 12v rail unless you have one of the beefy ones that requires a 6pin connector..
What is your exact PSU?