One of the ways having a credit card (or store card) can improve your rating is that the financial institution might say "Well if Visa trust them, why shouldn't we?" and you get some brownie points.I worked as a programmer for a bank and got to see their credit scoring process for someone applying for one of their credit cards or a loan. You even got a more positive result if you had a Mastercard and a Visa instead of 2 Visa's for example because this meant two institutions approved you.
Based on what I saw, I wrote a program that will give you some idea of what you're asking. Here's a free way of getting some idea of what your credit rating would be and how to improve it. It uses the same style of scoring process as the banks but becuase you are answering the questions (not equifax) there's no charge. It'll also work out you estimated credit limit.
Direct download :
All I ask is that if you do decide to get another credit card, please check out the American Express one I have advertised :)
Thanks, Michael