The R9 270X's performance is ~150% of that of the 750 Ti, however, it REQUIRES 2x 6-pin PCI graphics card power connectors where as the 750 Ti get its power via the PCIe slot.
Technically you can use a molex (or dual molex) to 6-pin PEG adapter for the 2nd 6-pin PEG connector if your PSU only has 1, but that is usually not advisable, because odds are that the PSU is too weak in that case.
You can have a look here too for the 750 Ti's & R9 270x'spower consumption figures:
http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_750_Ti/23.html
What PSU do you currently have, and what is the rest of your PC's specs?
Also, what do you want to do with the graphics card?