You definately can not go under 5gr per pound.300gr for a 60lb,and 350gr for a 70lb bow.Anything less than that will be like dry firing the bow,and can make any number of terrible things occur.Primarily Damage limbs,and cams.If the bow is slow even with a light arrow,like the older wheel style bows,or anything shooting under 260fps.I would consider a little heavier arrow than what I'm shooting.But with the faster bows say270+fps I feel like you can get away with a lighter arrow.Alot of the guys have told you right about more energy,and less noise from a heavier arrow.But how much is enough?Honestly I always recommend 6gr per pound.So for a 60lb bow that would be 360gr,and for a 70lb bow that would be 420gr.That is a perfect blend of speed,and energy.I was proving a point earlier,about how light you can go,and still have a very effective arrow.If you have a well balanced arrow(f.o.c. around 9-10%),that is around 6gr per pound,and have a sharp broadhead.You really don't need to worry about anything else,except where to put the pin.I shoot light,but I know alot of people are leery of being too light.If you would like I can help you find a set of arrows that will match your bow.Or you can check this site out they have an arrow weight chart for alot of todays arrows.It also has a f.o.c. calculator,and ke calculator.Hope I helped.

http://home.att.net/~sajackson/archery.html