Recommended Watercooling Kits
The following are common recommendations for entry-level watercooling. Most are aimed at the beginner, but all offer very good to exceptional performance for the price. They include everything you'll need in a single box...just add water. Most, if not all, have the ability to expand into larger, custom loops capable of supporting multiple radiators and the addition of one or more GPU blocks.
XSPC Rasa RS/RX 240/360
Older XSPC CPU-only kit utilizing their RS and RX series radiators in 240/360 versions coupled with their X20 750 (750 l/h [12.5 lpm]) bay/pump/res combo. There are also 120 versions, but they utilize the weaker X20 200 (200 l/h [3.33 lpm]) and aren't typically recommended except in very SFF cases. The X20 750 is a decent flowing pump and good for a CPU only loop, or even with a video card block and additional radiator, but once you start to consider multiple video card blocks, you should likely consider a better pump.
These kits are easy to setup and maintain and have used 7/16" ID tubing and fittings out of the box. The Rasa block is a very good performer in terms of CPU blocks, and the RX radiator series is excellent for price and performance. The RS rad series has since been replaced with the EX series rads, but do offer good performance for the price. It should be noted that there are updated versions to these kits that utilize an XSPC bay res combo with either a D5 or DDC pump (for additional cost).
Rasa RX360 http://www.xs-pc.com/products/watercooling-kits/rasa-750-rx360-watercooling-kit/