4 of these TECs at 12v would get you close to 0c on the cold side. They are 40x40mm so you'll need an ~80x80mm cold plate made out of copper to house all 4 of them right on your CPU, that needs to be about 10mm thick and should taper down from the 80mm to the size of your IHS. They'll handle 240w at a delta of about 30*c from your hotside temp, so you'd go close to zero on your cold plate/IHS, with all the thermal losses counted and stuff you'd probably end up with a real world CPU cold plate/IHS temp 20-25*c cooler than your water block is at now, maybe 25-30*c cooler if you're lucky.
The big barrier is getting the custom water block and cold plate made up in copper to suit the size of the 4 TECs. The next cost will be your PSU to power the TEC's, at 12v 4 of them will pull about 336w. Other than that you'll have to insulate your motherboard etc as you'll most likely have issues with condensation, unless you can track down a PWM dew point controller or have the wherewithal to make your own controller, can be done with an arduino or a straight up custom job like mindchill came up with, good piece of kit that was.
Or if you are prepared to risk it you could grab a couple of these http://www.ebay.com.au/itm/231704893134?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
you'd need 2 of them as you'll be pulling 28amps through the 4 tecs, maximum of 20a through each controller, so 14amps per each controller is fine and it allows you to set a temperature and it will maintain your TEC cold side temp at your set temperature, if you buy them you need to ask them to customise the temperature setpoint range from the standard 10-40*c down to about -10 to 20*c which would allow you to run your TEC's all out at 12v or as warm as 20*c if you have issues with condensation. You will need to buy a couple of NTC 10K 3950 thermistors to go with it, I bought one of these and I'm using a cable thermistor with the sensor inside a bead, I've drilled that inside my cold plate so that it feeds back the temp to the controller and the controller controls the TEC via PWM to always maintain your set point, so you'll need to put a TO-220 heatsink on the mosfet with 14amps going though it. This won't automatically keep you above the dew point, so you either need to set it at a temp which you are comfortable will always be above dew point, and it varies widely so that may not allow you to achieve the greatest temperatures as it won't be that far below your ambient temp, or you can insulate the motherboard and pick a temperature of about 5*c that you know your TEC's can maintain even at 100% CPU load and the controller will just keep your cold plate/IHS at that temp.
Make sure you do a good insulation job on the motherboard and CPU socket though. This whole setup only adds 336w to your whole loop so I'd say your water temp should remain about the same so I think you have a pretty good chance of getting your cold plate/IHS temp in that 0-10*c temp range even at 100% CPU load. Its up to you whether that result is worth the cost.