I have read just enough about light vs. heavy arrows to be really undecided. It seems to some extent another Ford vs. Chevy, .45 vs. 9mm, plain vs. serrated edge etc. argument. I suppose the place to start, at least if you intend to hunt is with your state hunting rules, here in Idaho for big game the combined arrow and broadhead must weigh at least 400 grains. I remember when carbon arrows came out and it seemed everyone wanted as light an arrow as possible because it would shoot flat, but then various gadgets to add weight supposedly to increase penetration started to show up on the market. Now there is a company in Alaska offering carbon/synthetic arrows that depending on the type of broadhead used weigh up to 850 grains or more for big, big game like bears, moose and African game. What I did when I started was take the recommended set up from the archery shop and then tried a couple different weights of field points, but ended up back at what I started with. Again if you plan to hunt you will want to consider a field point in a weight that you can also get broadheads in, while there are field points as light as 75 grains I dont remember any broadheads that light, and looking in the catalogs most broadheads seem to start at 100 grains and you will probably find a better selection when you get to 125 grains.
Todd
Watchful put up some good links while I thinking about this. Most of those links seem to be in the weight doesnt make to big of difference to penetration camp. I have tended to ignore the argument, as long as my arrows are legal and shoot well I dont know that it matters all that much.