I would suggest not buying a "complete kit" as they will supply cheapest parts thrown in a box of what they think would best fit your system. Chances are the water block is el-cheapo, water lines are tiny, pump is weak, water res is a leaky plastic thing that is hard to fill and purge.
www.dangerden.com is a site I trust to order from. They have very high quality stuff and you get what you pay for.
You can get a small pond pump from wal-mart, you can get radiator from auto parts store or junk yard, good water lines from home depot (water pump too).
Chances are you will end up with a much higher quality water rig than you would normally find as a "kit".
Water lines should be half inch ID for best water flow. Make sure you get matching blocks with half inch OD fittings, the same goes for the radiator.
<b>"Granted I dont own a P4. But I read enough stuff and waste enough time on forums newsgroups IRC and computer news sites that I proberly know more then if I DID own a P4." -vk2amv</b>