before i say anything...what i am going to say is purely just my opinion and what i have learned from my experience.
99% of the commercial cooling liquids out there are pretty much distilled water and ethonal glycol (EG) from what i remember EG is just a mild lube and anti-corrosion agent. if theres anyone that thinks anti-freeze is just for extreme high and low temps then they should look up the MSDS for all the commercial WC liquids. THEY ALL HAVE EG!!
they amount of anti-freeze you use should be w/e you like. i saw no temp difference between 100% anti-freeze and 10% antifreeze. only reason im doing 10% instead of 100% is because im out of AF.
if you are going to run distilled water and AF for a long time then you should buy yourself biocide to keep stuff from growing in your loop and gunking up your waterblocks tiny little pins.
make sure that your loop is completely sealed and there is no air venting from the loop because smelling AF is possibly bad for your health..hmmm w/e w/e
oh one last thing...this is possibly the most important thing about distilled water and AF...im pretty sure that the water is going to pick up electrolites from the different metals in your loop and im about 100% sure that AF has alot of electrolites in it in order to keep the water from boiling/freezing. soo...leak check first
Just my 2 cents, water on it's own is pretty much the best at absorbing heat. Antifreeze does not make it's ability to cool any better, but it has 2 main uses. The first is obvious, it makes the liquid unable to freeze at normal winter temps. Secondly, it has some additives to prevent corrosion. However, these additives are specific for an engine - if it's an aluminum block it needs different additives than a cast iron block. Considering CPUs are generally coated Nickel - or maybe you lap it down to the copper - then it's again different. I don't know what the best additive is for a PC watercooling system to prevent corrosion, although I know Nickel is fairly resistant whereas copper is not.
So, if it's an ok antifreeze to use, then you can pretty much dilute to whatever amount you want the only real difference would be the quantity of additives and therefore the corrosion resistance.
Now when we're talking antifreeze it's a little bit different that normal cathodic protection. http://www.freepatentsonline.com/4873011.html:
"An antifreeze composition for use in aluminum internal combustion engines is disclosed. It is comprised of from 0.05 percent to 0.5 percent silicate, from 0.07 percent to 0.35 percent nitrate, from 0.2 percent to 2.0 percent phosphate, from 0.5 percent to 4.0 percent benzoate, from 0.1 percent to 1.0 percent molybdate, from 0.02 percent to 0.3 percent vanadate, from 0.05 percent to 0.3 percent triazole, and from 100 ppm to 5000 ppm organosilane stabilizer for the silicate. Organic amines and borates are excluded in the compositions and the pH of the compositions is from about 7 to about 9."
So basically you should get that type of antifreeze not the normal one for cast irons. As to amount, I can't really say but the less you use the more frequent it needs to be changed... but again, I don't have any numbers for you. Make sure when changing that you purge the system, tho... there can be small buildups.
(Just so you know, my knowledge is not in WC set ups, but I have taken classes in chemistry, thermodynamics, fluid dynamics, and hydraulics which is all very related to WC)