If you're not already using Steam Guard, use it. It pings your email with a confirmation code whenever you (or someone else) tries to logon Steam from a different computer. In fact, Steam Guard actually kicked in when I first tried to use Steam after installing a different motherboard in my PC, so it definitely works.
I don't remember the exact procedure to set it up, but you can find it in the Steam client in View > Settings > Account.
All sensitive information like passwords, cards and so on were encrypted, "hashed and salted"* so in theory it should be fine. Going to keep an eye on my debit card statement over the next few days regardless.
*Insert Gaben Fat Joke Here
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magic sucks
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