3 ohms, 35 watts will get you 3.33 amps to the motors. 2.5 ohms gets you 3.92 amps to the motors. All this according to StepperCalc.
www.e-zflight.com/files/stepcalc102.zip
You can't drive these motors off the IC, you will need transistors or FETS to drive them.
The above calcs. assume your using FETS for the outputs.
Eric