Good thing the credit card info is all encrypted, and if you change your password you should have nothing to worry about.
Yep. If you ever use a credit card online to buy anything in this day and age, with any company, you are at some risk of this sort of thing happening. Just part of the modern reality. Pro tips for safety when using Steam: always keep the Steam Guard Account Security setting turned on- then nobody but you can change your account password- to do so, an email is sent to you when you start to change it, and you have to use the enclosed security code in order to complete the pwd change. Secondly, to avoid any remote possibility of your credit card number getting accessed and decrypted by hackers, simply choose not to save your credit card account number or info on Steam, when you use it to buy anything on Steam.
With those simple steps, you really have nothing much to worry about from these kind of events. Lastly, if your credit card info ever did get stolen, you do not have to pay for any fraudulent charges made to it by thieves- there are excellent consumer protections on all major credit cards, for your protection. They remain the safest way to purchase anything, online.