Go over to www.xtremesystems.org
and they will teach you everything you need to know.
Is this your first one? I recommend reading Nubius' post in the overclocking section on watercooling. It is a sticky, right at the top. If this is your first, I would build your own. I would buy a set, prolly a cheap one, and set it up and learn. Then for your second or third you can go to peicing together your own.
The larger the piping, the more flow, the cooler your system. Get half inch fittings on everything, and then use 3/8" tubing, so you have an increase flow rate. But like i said, you should buy a kit first. How much money are you looking to spend?