Picking the best Pump and Rad.

So I've seen many posts on what pump and Rad to buy. But what should I look for when buying? How many GPH should the pump do for 3/8 and 1/2 tubing? And what do you look at when picking the best Rad? Is 1/2 tubing better because it can move more water or are there other things to take into consideration? I'm thinking about getting the Swiftech 6000A CPU block if that helps.

What kind of case do you have to fit this stuff into? If you have the room, get a heatercore instead of a rad. I got my single pass heatercore at autozone for 17.99, and they are supposed to be the best (or one of the best) heatercores for watercooling.

I'm currently building a case. It is two mid tower case side by side. There will be enough room for anything I throw at it. Basically looks like a server style cube case. Can you explain more about a heatcore? Maybe some links to products?

If you have the room for anything, I suggest the Fedco 2-342. It's a single pass heatercore and fits 2 120mm fans on one side.