The best comparison i could find for you was on Tomshardware cpu charts.
I found this:
Supreme Commander Benchmark: (both on Intel P35 chipset)
E4300 1.8GHz 800FSB 2MB = 24.1 FPS
E2160 1.8GHz 800FSB 1MB = 16.0 FPS
Quake 4 Benchmark:
E4300 1.8GHz 800FSB 2MB = 79.0 FPS
E2160 1.8GHz 800FSB 1MB = 74.3 FPS
Serious Sam 2:
E4300 1.8GHz 800FSB 2MB = 100.4 FPS
E2160 1.8GHz 800FSB 1MB = 88.9 FPS
So as you can see, L2 cache does play a somewhat significant role in some games when comparing equal clockspeeds.
......of course the video card you buy will make the biggest difference so spend more on the GPU than the CPU!