If you're a hardcore gamer whose enjoying stunning graphics then in my opinion, core i5 would not do any good; but for casual gamer enjoy just time killing games for now and then. My suggestion is the 3rd generation Core because gaming speed is more important, while the 4th generation used lesser energy (system less hotter on extensive performance).
Turbo boost allow the core to process faster than its minimum, that why sometimes computer get extremely hot because of the turbo boost due it requires more energy to process the system data.