As soon as you start keeping coins in your wallet, you should enable encryption on it by clicking Settings -> Enable Encryption, and choosing a secure password. Then, make sure to regularly back up your wallet by clicking on File -> Backup Wallet. If you lose your hard drive and don't have your wallet backed up to someplace secure, you'll lose all your coins.
Unfortunately, you probably won't be able to make much of a profit mining altcoins. New ASIC boards are being designed to target LiteCoin and other Scrypt-based altcoins, and they will force even high-powered GPU mining rigs out of competition. For now, though, mining is an interesting way to learn about a technology that could be most important invention of the decade, without putting your retirement fund on the line.
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