The i7 series has hyper-threading.
There aren't any games that will make use of hyper-threading. In fact, the most cores a game currently utilizes is 3. The increase in performance from using 2 cores to 3 cores is not significant; about 10% - 15% depending on the game. Of course, if the game cannot use more than 2 cores then the improvement will be 0%.
If you know that the programs you will be using will take advantage of hyper-threading, then buy a i7 CPU. Other buy a i5 CPU.