Gigabyte Z68 is aftermarket, not 3rd party OEM like Dell, so you are good for anything upto a i7-3770k or any of the E3v2 Xeons. I'd not recommend anything less than a i7-3770k or Xeon 1270v2, which aren't much better overall than your current i5 except that they offer Hyperthreading and upto 8 thread usage which can be a very large bonus for games like highly modded Skyrim, gta5, Battlefield 4+etc which make good use of heavy thread count workloads. The higher clockspeeds will help simpler games like CSGO or LoL that use less than 4 threads.
The i7-3770K ability to OC easily generally @4.5GHz to 5.0GHz only helps everything.
Just be aware of costs of that cpu vrs a newer, budget platform that's it's equal (if not better) like the Ryzen R5 1600 AF on a lower end B450 mobo with 3200MHz ddr4, which runs @ $250+