I have two credit cards, my first card had a limit of $1000 which I got while in college. Today after five years they have raised the limit to exactly $10000. The second, American Express Gold, which I don't use at all, but just to keep it active I have linked it to my cell bill. Even though the AMEX card is not the oldest, I am afraid of canceling it because I think it will lower my credit score! My credit card balances do not exceed more than $200/month.

Should I cancel my AMEX Gold card? I think that one credit card is enough. If I charged to the limit they have given me, I would be in a financial nightmare! I have never had problems with Bank of America and they seem to raise my credit line every 6 months, so why bother with another card? So is one card enough to built credit? I really don't want to keep two cards and pay $100+ in membership fee!
Additional Details
Thank you all for taking the time and submitting your responses, I have read them all. Let me elaborate more. Some of you said that more than one cards are needed to establish good credit (at least 3). If someone has three cards with with $5000 each, and another individual has a single card with $15,000 then doesn't both of these individual have the same credit worthiness? Isn't having a single card the same as 3 the other individual has? Thus I ask isn't one card the same as three?

Since they have given me $10000 in five years I have no need for another card. I don't spend more than 200 per month on credit and I carry a balance once in a blue moon to keep the card company happy! I don't like plastic.

My first card does not have a fee, I got it while I was a freshmen in college. Within a year and a half AMEX started sending me offers, and since I was too dumb I picked the gold card, the funny thing is there is not an atom of atomic # 79 in the card! I guess I'll keep it for now.