Getting a quad core i5 CPU would be ideal if you can afford it. Most games still only use 2 cores, however, that is a very general statement. The games that you are interested in playing may support 4 cores and therefore a Core i5 CPU would provide better performance than a Core i3 CPU.
Playing games with a dual core CPU like the Core i3 is still possible even with games that can make use of 4 cores. However, doing so would mean lower performance. For example, lots of people play games on laptops however, only the Core i7 CPUs with the MQ, HQ and MX designations are quad core CPUs. All other Intel CPUs are only dual core with hyper threading.