We need to know a few other things.
1. What kind of monitor / resolution you are using or if you plan to get a new one soon?
2. How much do you want to spend?
3. What is your current power supply situation although this is less important as the newer gen of cards for nvidia are very power efficient and the gtx 780 probably will draw more but you may need different connections for newer graphic cards in case you have a older power supply as I don't recall what a gtx 780 would needs?
In either case a 1050 ti would probably not be a massive upgrade from a 780, could even be a downgrade, as you would need something a bit more powerful like a gtx 1070 to make it worthwhile over a 780.