I never even looked at AH before of a car battery. I figured they would be all around the same fort a specific size battery. I suppose that isnt really tue if the CCA can be different in the same size, means different material construction and so different capacity too.
I just looked up the battery I have which has been the best battery I have used so far. These are the specs on it:
Pulse (5 second) Hot Cranking Amps (PHCA) 1500
Cold Cranking Amps (CCA) 850
20Hr Nominal Capacity (Ah) 68
10Hr Nominal Capacity (Ah) 62
Reserve Capacity Minutes 135
So just based on the AH capacity of mine, I would think a 75AH battery should be plenty fine for your car? I cant imagine a 2003 has more electronics than a 2006 that I have.