Phil,
I do not have the specs for myDAQ, butif they are similar to or better than the 6008, then the same prinicples apply.
A 4:1 divider would give you better resolution provided you do not exceed the maximum input values for the device. The reason you would get better resolution is that you would be using more of the full scale range of the A/D converter.
You could use an op-amp for input protection but the DAQ input is probably protected better than most op amps. Overstressing the inuts of op-amps may destroy the devices or make them much noisier and change input offset voltages and other parameters. Generally an op-amp is not well suited as a protection device.
I suggest back to back 10 V zener diodes across the output resistor of the voltage divider as the protection. They are completely passive so no power supply is required and the input resistor of the voltage divider serves double duty by being part of the protection circuit. The zener voltage should be about a volt higher than the the highest instantaneous voltage that you want to measure and should be no more than 0.5 higher than the maximum permissible input voltage to the DAQ device.
Lynn
