Dielectric grease, and a LOT of it, it wasn't truly necessary because it has a heating pad on the back of the motherboard, but I didn't want to take the risk.
What CPU do you have again? Athlon64 I guess, put a lot of dielectric grease in the socket, cover the entire surrounding region of CPU, let's say about 7-10cm with a layer of dielectric grease. Put dielectric greaseon the side of the CPU once it's installed, top of the CPU with thermal compound, so on the CPU, it's either thermal compound or dielectric grease, there shouldn't be a single spot that isn't covered by something.
My Prometeia Mach2 isn't the best insulated version out there, in fact it's the ST version so cut down, doesn't have USB temp display or the best insulating system.
Oh yeah since you're using a chiller to cool your water, make sure you insulate the tubings, you'll see a significant temp drop if you do that.