Like the guys above said, the GeForce GTX 750 Ti doesn't use that much power and certainly doesn't need 600W unless you're running some really power hungry stuff on your PC that you didn't mention. It only requires one 6 pin power output from your PSU, as compared to the MSI GTX 560 Ti that needs two 6 pins. I myself use a 550w power supply to power the 560Ti and that's more than ample.
I suppose your Cooler Master Thunder could support this card, however it's not a great PSU, but if you don't want to spend $ you could just stick with it, else you can consider a cheap but decent option like the Cooler Master G550M