Any gpu upgrade will probably require a psu upgrade too unless that 750ti is seriously overpowered. So a gtx970 is looking at a decent 550w or better. Part of the issue currently is that cpus haven't really made any huge leaps in performance, so your board will handle upto a i7 3770k, which is still a very respectable cpu for gaming, as is the i5-3570K. Even as old as they are, there is still a demand for those cpus, so prices are still high, around the cost of much newer cpus, so the only advantage for the Lga1155 is not having to replace the motherboard, just the cpu. A new, decent lga1151 board will run about $150, last gen lga1150 Haswells run around $100+, so definitely a budget decision. If you want to try your luck on eBay (I did, got a 3770k for $50 used) that would be a decent option for getting an older cpu to fit your board without paying ridiculous retail prices.
A decent gtx970 is @$280-320, and for that price, you'd be looking at an AMD r9 390 as well, which basically performs the same, but either would be a massive leap in performance and ability over a 750ti