No of cores doesn't really matter when it comes to Gaming.Most of the games that are in use today don't use more than 2 cores.The speed and performance of a single matters a lot.See here
The Sandy Bridge Review: Intel Core i7-2600K, i5-2500K and Core i3-2100 Tested - AnandTech :: Your Source for Hardware Analysis and News
Notice the fps that i3 achieves.
Cache memory is very important for gaming.I'm not getting into the details but that's how it is.A CPU with a bigger cache will be a good gaming CPU.
Multiple cores are used by multithreaded applications ie software that can use multiple no.of cores.Majority of the software that we see today aren't multithreaded.
Photoshop,3DS Max,Maya etc are examples for multithreaded apps.
Will check the specs of those 2 CPUs and will post my view here.And multiple cores can be used for multitasking.I mean you can run many big applications in the background at the same time.
Athlon doesn't have L3 cache which is an issue.And I think you can unlock the Phenom to a quad which will be great.Even if you don't, you will get better gaming performance out of the Phenom.