You should always be honest in an interview, lest it comes back to bite you on the arse. I have interviewed quite a few people in my time and there is certainly nothing bad about leaving a previous job because you wanted to climb the career ladder and earn more money and better yourself.
Rather than wait for the interviewer to ask you though why dont you appear more proactive and ask him/her about the companies career progression plan, do they advertise jobs internally, is there a specific pay scale which is implemented, how often will your work be appraised so that you can see how well you are doing etc..... that way they will know you are not only keen to work for them, but you are interested in the role you have applied for.
Why dont you also do a bit of research into the company, what they do, how many employees roughly they have ! This is always a winner :)