Perhaps the most frequently asked question in this forum relates to people wanting input and advice about what to buy for watercooling.
When considering the purchase of a watercooling system, you need to ask yourself what your goals are. If you are primarily looking for a quiet system you need different parts than if you want to shoot for maximum overclocking. Most people are somewhere in between.
Next you need to consider your resources. How much do you want to spend...how big a case will you be using...how much time are you willing to invest, and how important is the overall appearance of your installed system?
There is no such thing as 'best' parts to use, as the parts need to be tailored to each individual. I'll offer some suggestions for each of the above scenarios, and hopefully this can help people make decisions when they decide to break into the 'watercooling game'.
You don't need the highest-capacity setup if you don't plan on overclocking. Prefab kits from any manufacturer will be adequate. Koolance setups are popular, as are rigs from Danger Den, Innovatek and Swiftech. You can certainly assemble your own setup from parts, but you won't require the additional capacity that can come from higher-volume components, and larger pumps and higher-cfm fans will take away from your goal of silence.
Build the highest-capacity system you can. Generally you can do better with individual components than any pre-assembled kit. You can still have a very quiet system but you're obviously going to deal with a lot of heat, so higher cfm fans and high-flow pumps as well as quality waterblocks are required. Since the greatest overclocking potential comes from the CPU, GPU, try avoid having extra components (like NB or hard drive blocks) connected on the same loop as your primary devices - they will add heat, reduce flow and decrease overclocking potential.
If you have a small case, you're going to have more difficulty fitting cooling parts inside, and you'll have tighter bends which restrict flow. Unless you are willing to put a lot of effort into building custom parts you'll lose capacity unless you have a large-ish case. You won't be able to use enormous heater cores with the best capacity in a small case.
Eheim pumps are very good quality...1048 for smaller systems and 1250 for larger. Swiftech's MCP600 has gone through some growing pains and reliability concerns but it is thought to have stabilized now. Danner pumps are popular, and Iwaki are considered a 'premium' quality pump. While this certainly isn't an exhaustive list of good pumps, be cautious of a no-name or lesser-known pumps. You need to use a pump that has high enough flow levels (including head, or ability to pump upwards) and of course have low failure rates.
Waterblocks are half the heat-transfer for your setup, so make sure you use a good block, and one designed for your processor. If you're starting out fresh, go for 1/2" ID barbs or fittings for higher capacity. Danger Den, Swiftech, D-tek, Innovatek all make good blocks. Cathar's blocks are considered by many to have the best cooling available at this time, but they are pricey.
Resevoirs are an optional component. They are generally used to facilitate filling and bleeding air, but a T-fitting works as well with less expense. Make sure your res can be placed to as to not add undue restriction to water flow.
Tubing is an often overlooked aspect of many systems. Thick-walled tubing like Tygon or Clearflex will help prevent kinks and have the most resistance to evaporation. Cheap vinyl tubing (from Home Depot or other places) isn't as good for water flow, but will allow colored and UV dyes to shine more brightly. Go with 1/2" ID tubing for higher capacity.
Dyes and tints are popular to add a certain 'look' to your setup, especially with neon and UV lights. Keep in mind that many dyes include particles that don't fully dissolve, and these can accelerate wear on your system like little grinding stones. Some pump manufacturers won't honor warranties if you run dyes in their setups. Even using distilled or 'pure' water you will eventually have some bio-growth in your water. You can either replace your water periodically, or use bio-retardant products. Don't use bleach in your system...it's caustic on seals and tubing.
Heater cores are a type of very effective radiator, although they tend to be large and require some space. Black Ice make various sizes of radiators that can be used in smaller spaces and are quite effective but cost more money. A general rule of thumb is that thicker and bigger radiators allow more cooling, although they require a higher-capacity pump and fan setup.
Fans normally have a 'dead spot' in the center, so you use a shroud or spacer to keep the fan an inch or two away from the rad for best cooling. Generally 'pulling' air through the radiator is more effective than 'blowing' at it. Higher capacity fans = more cooling, but generally more noise. Keep in mind that when used with a radiator, the amount of pressure a fan exerts is often more important than the maximum CFM rating with no pressure.
There is only a small amount of heat (fractions of a degree)absorbed into the water from each component on a given circuit. Contrary to popular opinion, the order in which you connect your components will not make a large difference in your temperatures. Certainly water is coolest after it leaves the radiator...but if you restrict waterflow or introduce kinks by having excess bends your temps will be worse than a simple well-flowing setup.
You will achieve best cooling through your radiator if you have cool air from outside the case blown through it, rather than warmer air from inside your case. This often means either fixing the rad at the front of your case, or else designing some ducting to bring cool air directly to the rad. Don't forget you'll need exhaust fans to pull warm air out of your case (or use more ducting). You can use a fanbus or similar device to lower fan speeds to whatever amount of cooling your individual system requires.
One of the greatest falicies in pc cooling is that too much attention is placed on temperatures. The thermistors used in normal computers are not calibrated very accurately. They are normally accurate to within ~5 degrees...but you can NOT expect to accurately compare aircooled and watercooled temps, and you can't even expect to compare watercooled temps between different systems. Just because somebody stated in the group that their temps are 2 degrees lower than yours, don't get worried that something is wrong. If your system is quiet...your system is overclocked to where you want...and your system is stable, be happy. It will be very obvious if you make a major mistake somewhere.
Certainly this isn't the last word on how to choose parts for your system...hopefully others will followup with more advice and suggestions. This is intended to be a resource for watercooling novices so they can make some decisions for themselves, rather than simply asking the group...and hoping to be given a good answer.
Now that you have some resources to help you choose your components...don't feel shy to ask the group about any concerns or problems you have setting up or tuning your system.