Tanks Peter for your attention!
How far will be Arduino be from the PC?
I think the Arduino most far away is about 15 to 20 m (cable length) away from the main pc.
Do you really need all those separate Arduinos? (Why?)
Well after all we want to control 3 - 3way Valves for up to 64 times and measure 15 temperatures for 64 times. So reasons for that many little guys:
-->the amout of devices that have to be controlled
-->the amount of sensors that have to be measured
--> an approach of building autonomous controllers
-->a simple way of structuring (one Arduino for one unit of sensors and valves)
A USB connection for flashing is not an option, since we have to use ethernet (or another BUS system) for the communication and since its laborious to run around and flash seperatly such an amount of Arduinos.
I just found this Bootloader - TFTP approach:http://playground.arduino.cc/Code/TFTPBootloader1
I guess this Bootloader is one of your mentioned options?!