Assuming you are trying to upgrade the CPU in a laptop... just be aware that most laptops have the CPU soddered into the socket which makes them not upgradeable.
The main difference between the mobile Core i3 and mobile Core i5 CPUs is Turbo Boost. Turbo Boost (TB) is a like an automatic overclock in the Core i5 and Core i7 CPUs. The increased speed varies in Core i5 (and Core i7) models; for example, the mobile Core i5 2410m has a stock speed of 2.3GHz and a TB speed of 2.9GHz when only one core is stressed, and a TB speed of 2.7GHz when both cores are stressed.
In case you did not know, only the mobile Core i7 CPUs with the "QM" designation are quad core CPUs.
As stated by Toxxyc, the increase in speed will not really affect the performance in many games because most games are limited by the video card. The nVidia GT 525 is pretty weak and it will bottleneck the CPU. In games that are not graphic intensive, there would be some performance increase.