You can definitely get the GTX 750ti for medium settings for gaming if that is what you want to do. The big issue is the PSU like others have said. If you want a better video card, you should get a 600W or better power supply with at least 40Amps on the 12v rail.
For dedicated gaming, either look into ATI R9 280X (a rebaged 7970 GHz edition) or Nvidia 770 if you want to play high settings at full 1080p. If you don't want that high then at least an ATI R9 280 or an Nvidia 760 Ti.
While you are at it, you should put in an SSD drive for your main OS. Your computer will feel like it's brand new and it will operate a lot faster...no more delay opening things.
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