I’d recomend a 20-80 solution and the good antifreeze. There are two varieties with various inhibitors, and the 50,000 mile variety can break down in a year or two in a computer, running 12/365 or 24/365 allowing rust to form inhibiting flow, while the 100,000 mile organic variety (the Toyota/Lexus red stuff comes to mind) will take more than twice as long. 10% is really only sufficient to kill algae, but at 20% you get the corrosion protection in decent quantities as well. Also, bumping it up to 20 gives you a nifty little added bonus: it pushes the boiling point from 212 (pure water) or 220 (10-90) up to a whopping 270+ degrees. This means that if your CPU starts to overheat for whatever reason or if the pump fails or radiator fails, the coolant can hold alot more heat (in joules, not the specific heat which is highest with pure water) and may save your CPU. At 212-220, your CPU can get that hot without necessarily having permanate damage, but if the coolant vaporizes you can kiss your CPU goodbye, and probably the pump too. It’s doubly important to up the antifreeze concentration if using peltiers, and in that case I might even go for a 30/70 solution and get the boiling point over 300. While you’ll loose some cooling performance over pure water, you gain added protection, and it’ll even keep your radiator even cleaner.
Remember though, antifreeze isn’t a terrible coolant as it is and you could probably use pure antifreeze if you wanted to. Another option is to use an oil-based cooling system rather than water, which has the added bonus of being nonconductive (think a light mineral oil or maybe even some brake or power steering fluid). I don’t recomend this though: you’ll need an oil pump rather than a water pump, which is usually driven off of a power take off belt from an automotive engine, or you’ll keep blowing through pumps.
Oh yeah, and if you go 20% or higher you don’t need to go out and get distilled water. In fact, you could use some, erm, to put it politely “used beer” to fill your system and it wouldn’t hurt it one bit. Pretty slick, huh?
One other note: given a large enough heatercore, you could even completely do without fans. Another option (commonly used by car manufacturers) is to have a variable speed fan that adjusts speeds using a varistor to run faster or slower depending on temperature. You can also use two pusher fans given a large enough radiator, which gives some level of redundancy as well).
It really depends on what you’re trying to do. If quiet and trouble-free are the most important things, then I’d suggest a larger, externally mounted heatercore with self adjusting fans and 20% antifreeze. If sheer cooling is the most important thing and we’re in the realm of water only, then run it with very little antifreeze – around 2-3% (just enough to kill algae). If running peltiers, I’d definitely add 20-30% antifreeze as your temperatures will get much more extreme.