placeboFX is on the money with this for air cooling. Isolate the rack from the rest of the room, force cold air in and hot air out. Constant airflow is key, and you'll need to keep the humidity under watch as well. Too little and the air doesn't conduct heat as well. Too much and you get condensation on cold surfaces that can run down onto exposed parts. That's the approach I'd use.



If you're really intent on water for OC and noise... I'm assuming the former is the primary reason... with the amount of tubing you'd need you'll eventually have to deal with condensation and potential leak problems. One leak on the top of the rack and your farm is done. Same with drips on cold lines. You'll definitely need humidity control. I imagine you're air conditioned anyway, but if you're going to be using an open reservoir at all that's a concern. As for piping, just remember that the longer your pipe run is the lower your pressure is at the end. I'd use two larger flow pipes run lengthwise with the card racks, flowing parallel from cold over each card to a hot return, like this viewed from the top:





| ================= <------ Cold

|| || || || || || || || || --- Inflow Line

X X X X X X X X X --- Cards

|| || || || || || || || || --- Outflow Line

| ================= ------>Hot





The ends of the hot and cold rails would be capped so the flow is forced over the cards. Use 1" to 1.5" pipes for your cold and hot rails for each rack, junction each rack rail into 2" to 3" vertical master pipes. I would feed the master pipes at the center to even the pressure between the rails.



If you're going to use that model pump you might need two with the above setup - one on the hot in and one on the cold out of your radiator. I'd prefer something a little beefier myself for a single pump design. The main reason I say this is you'll have back pressure at the radiator and you'll want to keep the pressure up enough to get good flow at the far end of your rails. More flow = more efficient cooling, so long as you don't put too much pressure on the lines. A little testing is always good before mounting any heat blocks onto the actual cards. I'd build the rig first, give it a shakedown run, then after you're satisfied it's working without trouble and your pressures are where you want without popping any hoses, move the whole rig into position and then attach the blocks to the cards.



You can still plastic sheet the whole area off and AC that space to keep the humidity down and the ambient air temperature cooler. As for materials, PVC/plastics are easier to work with and won't condense as quickly as metal, nor rust.



That's my $0.02 for what it's worth.