FX is bad for ST compared to desktop Intel CPUs like Sandy Bridge-Haswell, compared to Core 2 Duo, specially at 2.5GHz, the 4GHz FX is good for single thread...
you are focusing on cache more than you should in my opinion, yes, it can help in this game, but you are still talking about a difference that could be difficult to notice,
all 45nm "Core 2"
e5300 16,8FPS (2.60GHz FSB 200 2MB l2)
e7300 18,6FPS (2.66GHz FSB 266 3MB l2)
e8200 22,1FPS (2.66GHz FSB 333 6MB l2)
3% GPU load sounds wrong, unless the game is frozen or something,
also you might be overestimating your GPU, the 3650 only had 120sps from the HD 3000 era (3850 had 320) and the 500MHz DDR2 makes the memory bandwidth really low ... even the 8600M GT mentioned earlier would have a clear advantage, or current Intel IGPs.