ahhhh the age old question.
there is no real way to suggest what you should set it to if that is what you are asking. it is basically trial and error. don't worry as you shouldn't damage your hardware unless you are voltmodding. basically if you make a mistake something else will give before it is damaged e.g. crash and reboot. if it reboots your original settings should returm.
ballpark figure for a 6600 on stock cooling i think is ~400mhz, although i have heard of more than one case of them reaching GT speeds on a stock cooler