Depends, are you talking about a CPU with no L3 cache at all vs. one that has it, or the difference between having 6MB of L3 on a quad core and having 8MB of L3 on a quad?
The Athlon II CPUs were Phenom IIs without L3 cache. The Phenom IIs did see a small performance advantage in games when compared to Athlon IIs at the same clockspeed and core count. So L3 cache does help to an extent for games.
As for larger quantities of L3 cache, that doesn't really help as far as games go. When testing an i5 2500k and an i7 2600k with hyperthreading disabled and clockspeed set the same as the 2500k, there was no performance difference in virtually all games. The only exception to this was StarCraft II, which showed a very small performance boost (2 or 3 FPS) with the i7's extra 2MB of L3 cache over the i5.