No, the 6600 is an overclocked version of the 6500. The 6500 is an overclocked version of the 6400. Since you can overclock any of them, it doesn't matter that much which one you take. The cheapest one is the wisest typically, but chances are you might get a higher OC out of the 6500 (Most likely not even the case though). I'd say get the 6400 and OC it to 4.0 Ghz, and even higher when you need it.