The first question that comes to mind is whether you're looking for ultra-quiet, maximum performance, or somewhere in the middle. Also, do you want to keep everything in the case (i.e. no external box for the rad(s), pump, etc.), and how willing are you to break out the power tools to mod it?
All that aside, here are my recommendations for the time being:
Diameters for everything: The bigger the better (to an extent
). Really can't go wrong with 1/2" tubing and fittings. Doesn't hurt to have 3/4" fittings on your pump if you can, even if you're only running 1/2" tubing.
CPU block: Whitewater or RBX. Basically the best performing blocks available right now.
GPU block: Silverprop Fusion HL or DangerDen Maze4 GPU. The Silverprop is all copper so you won't have to worry about mixing aluminum and copper in your loop, whereas the DD Maze4 does have Al in it. I'm also not sure of the current availability of Silverprop, however.
Chipset block: Don't bother unless you're planning on hardcore voltage mods to your northbridge that would require the extra cooling. Chipset blocks only add another restriction to your loop and really only perform on par with decent, quiet non-stock air coolers.
Pump: DangerDen D4 (a rebadged Laing D4), Swiftech MCP600, or Danner Mag3. The DD D4 will produce the highest flow and head, is 12v and therefore doesn't require a relay, but is the priciest (~$80US) and loudest of the three. The MCP600 is also 12v, is extremely quiet, and is ideally suited for high-restriction loops, but is not preferable for low-restriction loops due to it's low flowrate at low-restriction setups. The MCP600 comes in in the middle of the pack for price, ~$40-50US. The Mag3 is the cheapest at ~$20-30US, but requires the use of a relay (I believe), and that'll run you ~$20US. The Mag3's flowrate in high-restriction loops is nearly the same as the MCP600's, but has the benefit of a higher flowrate when used in low-restriction loops. It's also slightly louder than the MCP600 and occasionally has a leakage problem, but it's easily fixed in about 5 minutes.
Radiator: '86 Chevette heatercore if you're looking for a single 120mm radiator setup and want to do it cheap. These can require some cleaning up, however. If you want a single 120mm rad ready to go, I'd go with DtekCustoms' HC. If you're looking for a dual 120mm rad, the Fedco 2-342 is probably the best way to go for price-performance, but like the Chevette HC, it'll need some cleaning up. For something straight out of the box, DangerDen and Dtek both have dual HC rad's. Make sure you stay away from Black Ice radiators in any case, they just don't perform as well as a clean HC. Also, the dual HC's generally require case-mods and/or external mounting.
Tubing: Tygon's the best in terms of clarity, longevity, and resistance to clouding and discoloration. But it's expensive, $25 bare minimum for 10'. ClearFlex60 is the next best option, at about half the price, but it clouds and discolors a bit easier.
Coolant: 90% water (distilled or de-ionized) 10% antifreeze of your choice. Opinions vary *wildly* on which antifreeze to use and the ratio to use, but keep in mind that pure, unadulterated water transfers heat the best, so you want enough antifreeze to break the surface tension (a la Water Wetter, which I *do not* recommend you use), keep the creepy-crawlies from growing, and slow down corrosion if you use aluminum and copper in the same loop. If you're willing to part with a not-unconsiderable amount of cash and a slight decrease in performance, FluidXP can be had for $25US per 32oz. If you'll be using a dual rad and/or a large reservoir you're going to need to order 64oz. or more, as this stuff should not be diluted in any way. You will take a hit of ~1-2c over water/antifreeze, but the upside is it nearly non-conductive and non-capacitive. I say nearly only to keep the sticklers in the audience at bay: there is a chance it could do some damage if it spilled, but it's negligible compared to the probability of water/antifreeze damaging your components.
Reservoir or T-line: Comes down to personal preference, really. A dual 5-1/4" reservoir with dye-lite in your coolant sure looks purty and bleeds air fast, but it will add a slight restriction to your loop. A t-line is easier to fill, will perform a little better, but takes longer to bleed the air from the system (unless you're using an air-trap or fill your entire system underwater before installing).
Fans: Pulling air through the radiator fins is always preferable to pushing it. A push/pull setup will yield marginally better performance, but will also cause a bit more turbulence, and thus noise. Evercool 120mm fans are nearly silent if you turn them down to 7v, but aren't overly loud at full speed; look quite nice, are relatively cheap, and are widely available. Papst M1A's are basically dead silent at 7v, move more air than an Evercool at any speed (I believe...I'm sure someone will correct me if I'm wrong), but cost more. There are plenty of other fans out there to look into, but these are the most common that I've seen. I'll leave further fan research up to you.
Shroud(s): Doesn't really matter if you want to make it yourself out of duct-tape/cardboard/plexi/sheetmetal/solid gold/whatever or just buy one...just make sure you use one! Without a shroud you're only using ~30-50% of the surface area of your radiator!
Clamps: Metal worm-screw-type clamps provide the surest seal, but require care so that you don't overtighten them and damage the barbs underneath. Plastic "clip" clamps (like the ones at DangerDen) work just fine, but are more of a hassle to install/un-install. Zip ties work ok in a pinch, but I can't really recommend them. They just don't feel as secure as other clamps, and removing them can be precarious. You also run the risk of damaging the ends of your tubing as you generally need a razor/knife to remove them.
Phew, well I didn't really intend for this to get quite this drawn out, but what-the-hey. I'm sure people will agree/disagree with some, if not most, of this, but hopefully this helped get you started
