even though you do not want build recommendations there are a few things to consider:
i would highly suggest going with wd caviar blacks in 1.5tb instead of green 2tb. 5400 rpm drives are rather slow. also some wd hdds with 2tb sizes were rather unreliable a few years back (i had 3 which failed) so this is another valid thing to consider.
i would also highly recommend a samsung 830 256gb ssd drive. push to boot in 6 seconds flat on my laptop with sata III.
purchase a video card with a preinstalled water cooling block. if you do not i hope you do not value the warranty as it will be void in most cases.
with all of that said, here is some information on watercooling. opinions differ on this matter and everything i say here is pulled from all the research i've done on the matter. in the end i decided the cost far outweighed any potential benefit. in the end though it is better to be on the safe side then to underrate your cooling. house temperature/climate/system demand also come into play on final figures for cooling.
first realize that a watercooling system is not magical cooling and your numbers will not be much better than good air cooling. also realize that you can easily spend 1/4 to 1/2 as much as you did on parts on cooling accessories if you go this route.
i would NOT, let me repeat that: NOT try to put everything in one loop. the cooling performance will be terrible.
at absolute bare minimum i would have two loops. one with the cpu and two gpus. one with the mb, two gpus and other parts. alternating cooling bricks and radiators will improve cooling efficiency but still is not as effective as 3 loops see below.
a better setup would be to have three loops. one with the cpu and other parts. one with two gpus. one with two gpus.
high powered graphics cards are normally recommended to have 3x120 radiator space. you might be able to get away with 2x120 radiator space in the thick dense extreme radiators danger den makes. realize the flow restiction though. realize that you have 4 such graphics cards so fan space might become an issue.
you can double stack radiators at the cost of cooling efficiency provided you have a powerful fan.
i would highly suggest a large reservoir. the larger the better. a larger volume of liquid takes longer to heat up which requires your system to not work as hard. a passive cooling reservoir like the reservator (look it up) is not a bad idea.
you may also want a digital automatic fan controller or manual rheobus fan controller hooked up to all of the fans on your radiators. make sure the total wattages are not exceeded by the fans you intend to use. some high flow fans can draw quite a bit.
by far the best fan in terms of moving serious air are the ones made by delta. even at half speed (to reduce noise) they are quite powerful. at full blast they are quite noisy from the airflow not from the actual motor.
there is a pc watercooling forum around somewhere you may want to look at . i forget the website name though.
just some general recommendations and things to think about. realize that with good equipment you can very easily spend $2000 on watercooling instead of $300 on a good heatsink and better aircooling fans and a cpu heatsink.
You will not be changing out the coolant. Just buy enough to fill your system.
Are you planning on mounting fans to your case window? Where are you planning on putting the rads?
I would give dangerden a holler and ask for a recommendation for rads and pump velocities. Since they are just about the best you will be buying a bunch from them so I am sure their sales dept will help.
the problem with recommending a parts list is that it could change depending on how you plan on hooking everything up. here is a very generalized list to be modified:
-4x GPU blocks (i HIGHLY suggest buying a gpu with a premounted block to keep the warranty)
-1x CPU block
-1x NB block (northbridge fans are extremely loud so you may want to replace it)
-1-3x reservoirs. you can keep all loops seperate or use one large reservoir. you could also use an external reservator for some passive cooling ability at the cost of not being able to move around the system as easily.
-10x120mm extreme radiator surface area. this is a fairly conservative estimate. you may want to verify with dangerden.
-2-3x pumps. this will vary on your setup. this is another question for dangerden.
-tube clamps for mounting
-fillports or fill plugs on reservoirs
-flow meter (completely optional of course)
-coolant (can be distilled water or specialty coolant. avoid using dye or other addittives which could settle and cause clogging)
-case fans for radiators. if mounting radiators two deep you may need to buy a hefty fan or mount fans in a push/pull orientation (one fan on each side of the radiators). delta makes some hefty fans. i have one which will blow itself across the table at full power. they also can generate some good pressure for pulling through radiators.
-fittings (this will vary with what you end up creating as far as a layout is concerned)
danger den could also suggest a tubing size for you to use.
you can start looking at how many fan slots you have in your case i believe the listing said a corsair 800d which has 3 top, 1 rear and one side. you may very well have to window mount radiators. you could possible get two double stacked 3x120 on top, 2x120 rear which puts you at around 8x120 total. you would have to window mount one or two slots. keep in mind double stacking normal radiators is not suggested and double stacking extreme radiators is definitely not ideal in the least. this is why i say the whole setup can vary based on what exactly your loop and placement options are.
you can also start to get a rough idea on how much everything will cost. of course this is only an estimate currently.