The vast majority of games only use 2 cores. There are some games that can use more than 2 cores and more will eventually released, however, they will be small in number compared to the number of games only using 2 cores.
Naturally, you should research which games you plan on playing to find out if they can make more than 2 cores. For example, Skyrim only use 2 cores. Battlefield 3 only uses 2 core in single player mode, but it can use more than 2 cores in multi-player mode. I believe ARMA 2 can use up to 6 cores. Crysis 3 is capable of using more than 2 cores.
Let's take a look at Crysis 3 since it is new and I was able to find a CPU comparison chart as follows:
Source:
http://www.pcgameshardware.de/Crysis-3-PC-235317/Tests/Crysis-3-Test-CPU-Benchmark-1056578/galerie/2046280/#?a_id=1056578&g_id=-1&i_id=2047132
The Core i3-3220 @ 3.3GHz can get 31.7 average FPS. The Core i5-3570K @ 3.4GHz is only 100MHz faster, but the average FPS is 50.5. That represents a 59% increase in FPS; or 18.8 FPS. The 100MHz difference contributes a very small amount of that increase (like 1 FPS out of the 18.8 FPS). A quad core is clearly better.
The problem with i7-3630QM @ 2.4GHz and the i5-3380M @ 2.9GHz is the 500MHz difference. That's basically a 21% difference in clock speed. Naturally, that means you will not be getting a 59% increase in FPS, but something less. A rough estimate is about a 46% increase. That's calc'ed by 59% * (100% - 21%).
When you play games that only use 2 cores, then the core i5-3380M would perform better simply because of the higher clockspeed. Skyrim will run better on the i5-3380m, it will be less than 21% though (the % difference between 2.9GHz and 2.4GHz) because clockspeed does not scale perfectly in the real world.