Ok I'll jump on the bandwagon.
Buy a universal GPU block, because if you decide to change your GPU within a year or two, that $100 full cover GPU block you just bought is going to become a paperweight. Universal GPU blocks will last years provided you stick with the same red or green, though mine fits both. Just get some heatsinks for $20 and stick them on. I bought the Alpenfohn universal GPU heatsinks, and I have a crapload spare if I ever decide to do a second GPU or change. Plus they're black instead of typical silver, so they camouflage quite well inside my case.
As for liquid, it really doesnt matter, you can go clear liquid + coloured tubing, or clear tubing and coloured liquid (most of which, like the Mayhems(mine) comes prebiocided and treated etc etc etc).
I use barb fitting because I'm oldschool and I prefer the industrial liquid cooling look, compression fittings are the nice neat looking ones, but they're pretty expensive.
All of this depends upon your budget of course.
Dont buy nickel EK CPU or GPU blocks because they flake, not matter how much they have improved, they still flake as far as I know. EK do however make epic thick rads. I'm using one of their super thick 120mm Rads.
As a rule of thumb, use 120mmx120mm of Rad per item cooled, so if you're doing CPU+GPU+GPU, I recommend 360mm rad or 240 + 120 etc etc. Its always good to go for an extra 120mm worth to give yourself headroom.
CHECK YOUR DAMN SCREW SIZES. Too many times over the last few weeks I've been screwed over when trying to mount fans to radiators and radiators to specific areas of my case. Get the right thread size screw, and the right length screw.
In brutal honesty, it doesnt matter which way round you put your rads and heatinks, the temperature of the water will even out after 30minutes of flow, so it wont matter.
I went for a Res/Pump combo unit because there isnt enough case space for a dedicated Res and Pump area, but I have far too many 5.25" bays. If you have a serious case of too many god damn useless 5.25" bays, get a big old dual or single bay Res/Pump, the D5's are pretty good pump units.
Plan your tubing: The first setup I had with my watercooling had a kink between the GPU and 120mm rad, now I've moved it, its pumping better. Make sure the bends in your system arent severe, the wider the angle of the bends in the tubing, the easier the pumps life will be.
Ideally, if you're going for seperate res and seperate pump, try to put your res above your pump and flowing downwards, it just makes the pumps life 10 times easier at start up.
Make sure you have one of those PSU paperclip thingymajiggers, so when everything is set up, and theres no water in there, you can fill your res to the top, flick the PSU switch on and it will pump round without stressing the pump. When the Res is empty, turn it off, fill it back up, and turn it back on again.
Dont worry about air bubbles in thin/medium radiators, they should bleed out themselves.
I dont know what else. Try to think of watercooling as something to make your case look cool. Watercooling is obviously going to give epic temperatures, no matter how thin or thick your rads are and how good your SP fans far(anything with 2.0m Static Pressure in H2O or above is best), the whole point, at least for me, was to make the thing look awesome. So take your time, and save up enough to make the kit look exactly how you want it to. I spent £20 on 2 litres of white mayhem coolant because I wanted it to look cool, I didnt care if there was 1 or 2 degrees different between that and normal distilled water.
Work with what you have, dont try to ghetto mod water cooling kits, because more often than not it wont work as well as you hoped.
Also, dont worry so much about leaks. Tighten your fitting gently, not too tight, and make sure your hose is cut straight, and as far againt the fitting as possible and you'll be fine.
Make sure you buy at least 1 meter extra of tubing than you need. Watercooling makes you addicted to touching it, you'll want to move things around in a months time, or you'll fuck up the length, best to get some extra. I still have 1 meter spare even after todays tweaking, so if I'm feeling like I have too much money I might watercooling my GTX 275 Physx card for the fun of it.
If a tool like me can watercool and get great results and an awesome looking end result, I'm fairly certain anyone else can too.
