bowhunter,
If you are shooting 50# or less I'd recommend 5/16 birch dowels, if heavier then go to a 3/8 hardwood dowel. The 5/16 birch will give you arrows in the 450 to 500 grain range (w/ 125 heads) and the 3/8" hardwood will go in the neighborhood of 550/600 up to 700/800 depending on the wood type. You can tweek your spine by what weight head you choose and the actual length of the finished shaft.
One other thing, if you buy the dowels from the store you will pay a lot more than if you look up dowel rods on the internet a buy a hundred....ya can get the birch from 17 up to 30 dollars a hundred. hope that helps ya out some
rich