For just $70 it's a small upgrade, don't expect miracles, you'll see little or no improvement in older titles because they only use 1 or 2 cores, but a little more in modern, better threaded titles that can use the i7s Hyperthreading.
Also check your motherboard will accept the i7, IT is the Devils Canyon update of your own Haswell CPU, a BIOS flash may be required to support it.
And yes, as racksmith101 points out, you'll probably be able to recoup most of the cost by selling the current i5, although E-bay ( and its fees ) may not be the most cost effective way.