If I were you, I'd stay w/ 3/8" ID hose. You can convert the system to 1/2" some other time if you are unhappy w/ your performance. But as your first watercooling attempt, I would keep it simple. Grab a waterblock. Grab a pump. Get an Eheim one, because they are low-noise and high quality. Get 3/8" ID, 5/8" ID silicone hose from
www.mcmaster.com. This thick-walled stuff is super flexible and won't crimp. For a radiator, I'd probably get a Black Ice radiator because it will come fitted w/ hose barbs and mounting stuff for ya. Plus it is quite the performer. So that's a basic system right there. Get acquainted with it and read up about watercooling more. Eventually you can tweak the system w/ things like converting hose to 1/2", adding components to the cooling loop, etc.
-Kevin