It's more than just the cache. Here is a site that has a Athlon 635 vs. the Phenom 555.
http://www.anandtech.com/show/2923/6
In most cases, the Phenom II beat out the Athlon 635. This points more to just pure raw power (the speed of the cores) rather than multiple CPUs since most games aren't programmed for multiple threads.
This site has Crysis using the 630 and 555 and the 555 beats it by 10 fps:
http://www.legitreviews.com/article/1073/12/
However, the Athlon still does give some good framerates.
It might come down to price per performance. Which is cheaper and is it just for gaming you want? These are things you'll have to decide.
In then end, what is your breaking point for pricing? You might even be able to find a better X4 Phenom for a good price or even an X3.