Another item to remember when talking about surface area for picking up heat in a waterblock is that the amount of copper you are using in the first place determines how quickly large tempreture fluctuations can be compensated for.
More copper, the quicker the copper soaks up the heat from the heat spreader or core. The whole point of these extreme cooling solutions is not to make a pretty block or a small block, but to make a block that removes the heat from that CPU as fast as possible.
Copper will soak up heat until it's evenly distributed. When it has water flowing through it, it will carry the heat away. Even if that heat sits in the copper for a few moments, it will be OUT OF THE CPU.
The more surface area you have contacting the water the more heat you will ultimately transfer to the water and OUT of the block. The reason fins and pins work so well is that it increases the surface area that contacts the water during it's path through the block.
Presuming your radiator can get rid of the same amount of heat or more that your CPU can generate, then you will have a system that will stay pretty cool.
The block I made happens to be 2" x 3" x 0.5" solid copper and has 0.25" passages bored into it. I have 5 switchbacks in the path which increase my path length to approximately 10.6 inches. Because of that my total surface area inside block is about 8.3 square inches. That's more than the surface area of one of the large sides of the block itself (2" x 3" = 6 square inches).
Hope that helps you...
