If you're not interested in overclocking too high, the stock heatsink will do, but it doesn't hurt to get a good aftermarket cooler. I personally find the Zalman VF700-Cu to be very good, and there's no reason to think why the newer V900-Cu isn't better.
It doesn't vent out, so good airflow within the case helps. Apparently if you had good airflow within the case, performance is better than arctic silencer according to some reviews I read.