Hi there,
It really depends on the game whether a quad core is properly made use of. It requires the developers to code the game so it uses more than two threads for the CPU-intensive tasks, some do it better than others.
This means that in older games a dual core is often as good as a quad core using a similar design. However, increasingly games are making greater use of more cores - so having a quad core is well worth it if you want to keep your gaming system up-to-speed for some time to come.
At the moment the gaming CPU of choice is the Intel Core i5 2500K (alongside a Z68 motherboard so you can overclock the CPU). When Ivy bridge CPUs come out in a few months this position will be taken up by the i5 3570K.