I run mine with about 10% antifreeze no other additions...
Average?? Well today I got about 20-22C outside and about the same inside, about 10C down under the house (Spring isn't really here yet) and that gives me an operating temperature about 32-34C on my Dual running modded XP 1800+. Running a single case fan for venting the hdd's and such... Homemade blocks, an eheim 1048 pump and a huge industrial oilcooler (It's supposed to cool more puters later on...)
The tubing I'm not very sure about... rather soft silicone tubing... bougth in the nearest autoparts store, oil and fuel resistant...
I haven't tested this much during the "hottest" part of summer...ie it ain't very hot for very long here... but during the winter I had at the most -18c under the house... without any problems... although I wouldn't recommend shutting down the puter or pump at that time...