Athlon II X2 has 1-2MB L2 cache.
i3 530 only has 1/2 MB L2 cache.
L2 cache could explain the difference (but maybe not). The difference could also be explained by different motherboards, possibly different ram (ddr2 vs ddr3), updated gpu drivers ...
I think its more accurate to compare similiar or almost identical cpu's ... than to compare AMD to Intel.
As I wrote before, Fallout New Vegas never maxes out my i3 530 @4GHz+.
cpu and gpu usage are usually around 50% in that game.
A cpu which is slower, clock for clock, could possibly perform better than my i3 if it has more cache or better/faster cache ... if it is able to run at 100% cpu usage.
I am currently playing gothic 3 now and I also see around 30fps when I look at a large group of NPC's. I have also seen drops to 30 fps in Oblivion: Nehrim mod. When I fight spiders in Nehrim, fps drops to around 20. Spiders are fps killers in that game, for me.