I have a gtx 750 ti 1gb in my PC. Am I better off getting another 750 or getting an upgraded card such as a 770 or a 270x?

Solution
Correct, you can't sli a GTX 750 ti. Your gona have to get new card. And BTW... the GTX 770 and r9 270X are completely different cards, the GTX 770 will demolish the r9 270X.

I recommend a r9 280X instead, it's cheaper than a gtx 770 but equal to it.