Raiin :
a 960 is about 50-60% faster than a 750ti, anything 30% or more i'd consider worthy of upgrade.
That's funny, because somebody else said it was only around 20% performance increase.
Either way, the GTX ti FTW is $160 (got mine for $120 btw), and a cheap GTX 960 is only $200. So with the extra performance and the newer features the gtx 960 brings it's kind of a no brainer to buy that over the gtx 750 ti.