first of all, to get your temps lower, you will need to get the water down below ambient. as long as you're not under-radded, your liquid temp will be close to ambient already (most of the temperature difference is found over the waterblock... in other words, the difference between the CPU itself and the water)
for that, you'll need a lot of "Q" (the wattage of heat a TEC moves)... about 2 times your CPU output i think would give an optimal efficiency and temperature, maybe 3 times. you cant do that with a single TEC, which means you'll be needing dedicated chiller blocks that fit mutliple TECs. these arent widely available, and not available at all in copper (usually theyre made from aluminum, and im sure you already know aluminum in watercooling causes galvanic corrosion which poses a heavy threat to your cooling system) so that means you'll need to have someone make you a custom chiller. someone with acces to a milling machine. then those TECs need power. depending on which model TEC you use you'll need either a 12v or 24v dedicated power supply. ATX power supplies need to be modified to a single rail in order to work, and need to be capable of delivering the necessary amps from that single rail. (only look at the 12v rating)
then you need to insulate your cooling system. even if your CPU wont go below ambient, your tubing and blocks will, and condensation can still drop off those onto your precious hardware. (and as said, if you dont plan to get the coolant below ambient, TECs are not worth it over a big radiator.)
in other words... TEC's are expensive and a lot of work. fi you want to go trough with it...
you'll need to determine the power you'll want to move... the Q... around 2-3 times your CPUs heat output. thats around 600-800W.
all TEC's are rated for a certain Qmax, the Q they move at Vmax, their maximum voltage. but we dont need Qmax, it doesnt tell us very much about the point we're actually going to operate the TEC at... its just an easy way of identifying different models. for example, when people speak of the 437W TEC, than theyre talking about the model with a 437W Qmax rating.
most TECs run at 15.4 Vmax, but the lower voltage you apply, the more efficient the TEC becomes. (for example, the 320W TEC moves 320W at Vmax and consumes about... 450-500W (guesstimate, dont know the exact numbers from the top of my head) or so doing that. while if you turn the voltage down to... say.. 7v, it may move around 150W, and consume about 120W in the process... again, just a guesstimate but it illustrates my point) however, when you turn the voltage down, so goes the maximum temperature difference a TEC can generate. so that means there is a sweet spot... around 10-12v is a nice spot for most people.
so lets say you want to use 4 TEC's, and plan to run them at 12v... that means they need to move 150-200W each. that means 2 nice models are the 226W and the 320W models, depending on whether you take the 600 or 800W total Q figure. i think around that point, the TECs themselves will also consume around 600-800W... maybe a little more... a good single rail 1KW powersupply should run that. (PCP&C, Silverstone..) but make sure you use as many power leads as possible... connecting a TEC to a single molex will melt the cable. just like a high end video card, it needs multiple wires.
now, you see that the TECs move 600-800W, and consume (and therefore produce) another 600-800W... which means your radiator will have to deal with 1200-1600W of heat in a worst case scenario... you'll need a big radiator, maybe 2.
then you need the chiller blocks... a block for the hot side and one for the cold side. those two blocks need to clamp the TECs strongly between them... i think 300PSI was the optimal pressure but i may be confusing units... so dont quote me on that.
then theres the problem that you cant use acetal/delrin on the cold side loop... it turns brittle when it gets cold... not sure at what temperature it does that, but i suggest using a different plastic, or a metal like brass or copper. (duniek for example makes nice brass and copper tops for the swiftech GTZ) im not sure if acryl would work.
tehn insulation... lots of neoprene, kneadable eraser/liquid electrical tape, dielectric grease/vaseline for the motherboard... foam tubing insulation... some foam for the chiller blocks, at least for the cold side, but recommending both sides at once.
gives you about an idea what you'll need to have and do..