you wont get much difference from upgrading the g3258 to i3 skip the i3 go to i5 if you cant the i3 is waste of money many people would disagree but the only difference b/w the i3 and pentium g is Hyper-threading so no it isn't worth it just get an i5 when you can.
Secondly the gtx 750ti vs 950
the improvement will be mostly 10-15 frames is the improvement worthit as you would spend 160$ ? Nope
So what to do ?
Save more money and get i5 4690K or i7 if you can (i5 should be enuf).
Another thing: No dont worry about them getting out of stock soon remember that even today you can get i3 1st gen.