Benchmark with the Q9550 and i3 2100. The 2120 will be a few mHz faster. As you can see the 2100 performs better overall. The video benchmarking does improve ever so slightly with the Q9550, but considering the 2120's speed improvement, it might become even.
The i3 uses a DMI instead of a separate front side bus, so you should see overall performance gains just from the architecture.
so an I3 2120 would be the best choice? i was thinking that a 4 physical core ( for Q9550 ) would be more efficient than a 2 core + 2 thread ( i3 2120 ). any thoughts with this? sorry for a bad english
The Q9550 is older and uses a different architecture than the i3. It's difficult to make a direct comparison unless you look at benchmarking.
With that said, you should base After Effects multiprocessing off of physical cores. with the Q9550 you may be able to set multi processing to 2, but with the i3 you would be limited to one core (might as well disable multicore). How much RAM you have may actually be the deciding factor in optimizing that along with what effects you use.
AE multiprocessing tends to be buggy or counter productive when using any time remapping or noise reduction; using effects that reference or calculate off the results of other frames. AE multiprocessing, in some instances may be slower than allowing the CPU to use all the power on a single frame at a time... really depends on what you're working on as well.