Kind of confused about fitting sizes on my setup...

Basically I'm going to get MCP600 pump... it says 1/2" barbs does that mean ID or OD? The heatercore I plan on getting has 1/2" OD barbs which I'm guessing equates to a 3/8" ID? it's the black dual heatercore from voyeurmods.com.Anyne know if the fittings on there are chaneable? I read on here that the flow will only be as good as the smallest diameter so does this mean everything else should be 3/8" ID too? Is 1/2" ID a whole lot better than 3/8" ID? Sorry for all these dumb questions but before I take the plunge i'd like to get all my questions out of the way. Thanks for the help guys!

Tubing fits over barbs, so 1/2'' barbs requires 1/2'' ID tubing. Yes, 1/2'' OD tubing usually equates to 3/8'' ID, and the smaller tube diameter will restrict your flow more, but not as badly as an entirely 3/8'' ID setup. Make sure your pump inlet is not being restricted by 3/8'' ID hose, or your pump will have trouble. I looked at that heatercore and I don't see the barb sizes...but you might want to look into a different heatercore because the 3/8'' will hurt flow. The Double Heatercore from Dangerden.com is 15 dollars cheaper, and gives you the option of 1/2'' or 3/8''.