The single, most deciding factor in determining whether or not you get that mortgage or that car loan (and how much interest you’ll be paying) is your credit score. For years, the score has been calculated using the same rules but now that’s about to change because Fair Isaac Corp, the private company responsible for calculating your score, is changing those rules.

How will it affect you? Well, that depends on your current credit ranking and your credit habits.

Let’s look at some of the changes.

Types of Credit

Raising your credit scores with the new FICO 2008, you need to have a diverse credit portfolio. A wallet of credit cards won’t be sufficient. You will score points for having a mix of loans, mortgage and auto specially, and revolving accounts like credit cards.

The rationale is lenders need to see you can handle a variety of types of debt before they feel comfortable trusting you with additional money.

Credit Limits

Having credit cards is, therefore, a good idea. However, using them responsibly is a must. Your credit score will go down if you use charge too much of that available credit. If fact, to be safe, you don’t want to maintain a balance of more than 50 or 60% of you credit limit.

Lenders want to make sure you aren’t piling on too much debt. A borrower who already has three or four maxed out credit cards may be sinking themselves into a hole they will have trouble digging out of and lenders don’t want to be part of that experience.

Applying for Credit

In the past, your credit score could be hit pretty hard if you tried applying for credit from many different sources. That looked as if you were just trying to take on as much debt as you could your hands on or were being turned down a lot for being a poor risk.

One of the rule changes, however, is that shopping for credit from multiple sources won’t hurt nearly as bad. It’s still not a great idea to overdo it, but looking for a good deal by applying for the same type of loan, for example, is now acceptable.

Late Payments

FICO used to have no mercy when it came to delinquent accounts. If you were late, especially late more than 90 days, then your score would plummet like it had reached the highest hill on a roller coaster. That’s not necessarily the case anymore.

The new scoring system is a little more forgiving. If you occasionally miss a due date, don’t worry. If you’re 90 days past due on a single account but all of your other accounts are current, then don’t worry. However, if you are frequently late with your payments or are seriously delinquent on multiple accounts, your credit score will drop considerably.

The bottom line is the new FICO scores are meant to ensure responsible credit users get a break when accidents happen. They are also meant to keep people who are already over their heads in debt from getting additional credit from lenders.

You can expect to start seeing these changes reflected in your FICO score slowly throughout 2008 as the new rules are adopted by each of the three credit bureaus.