The best performance will come from low latency PC6400 I think... there's a Tom's article that roughly covers this detailing the intro of Socket AM2. Its
here.
Later on in the benches they sort of do a comparison with different RAM speeds so you can see the trade off. Unfortunately they only seem to do bandwith differences and not real world differences. My guess is that real world will have less pronounced differences with the different RAM speeds, but I've been wrong before.
-mcg