Assuming the same clockspeed, the performance increase to Core 2 Duo/Quad to the 1st gen Core i3/i5/i7 CPUs is on average about 10%. Going from 1st gen to 2nd gen Core i series the increase is about another 12%. Lastly Ivy Bridge represents the 3rd gen Core i series and is about 7% more powerful than the previous generation.
Assuming you are buying an Ivy Bridge 2.5GHz Core i3 CPU, it will basically be about as powerful as a Core 2 Duo CPU @ 3.26GHz. The overall impact to games should be none to relatively small. Games that are not CPU dependent will see no increase in performance like Crysis 2. Games that are CPU dependent should get a small increase in performance like in Skyrim.