Hey GTA,
I've tried both...
1. Placing a TEC cooled waterblock right before your CPU waterblock will act as a chiller and drop the water temp a few degrees. If you are air-cooling the hotside of the TEC it will take a big heatsink with a high CFM (very noisy) fan. The amount you will be able to cool the water will depend on your amb air temps and how cool you can keep the hotside of the TEC. I was dissipointed in how little (about 3 deg C with 172 watt TEC) it atually dropped my CPU temps.
2. I used two 226 watt TECs sandwiched between two Maze2 waterblocks and a Maze3 on the CPU. One E-1250 pump, two heatercore rads in parallel and 1/2" ID silicone tubing.
pump > Rads > TEC cold side block > CPU block > TEC hot side block > Res > pump
It actually made the water hotter...
This really drilled home the point of how much heat the TECs themselves dump into the system. Flat didn't work. You have to use two loops: hot side and cold side.
IMHO TECs are not very efficient and are very expensive (with dedicated PSUs) when used as water chillers.
Good luck,