You're right -- computers certainly are powerful, and getting more so every year. When I was a boy growing up on my father's dairy farm, computers hadn't even been invented; today computers would probably run it!
Could they take over the world some day? Computers can be used for great good -- but we also are discovering that they can be used for great evil. Who knows what might happen if an evil tyrant or maniac tried to hack into our major computer networks and turn them against us? Perhaps this will become the new front line in our battle against terrorism.
But computers only do what someone tells them to do -- and that's why your question is essentially a spiritual one. Our real problem isn't just what some evil genius might do with computers some day. Our real problem is the human heart and its terrible capacity for evil. Jesus said, "Out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false testimony, slander" (Matthew 15:19). This is why we need Christ, for only He can cleanse our hearts and make us want to do good. Have you given your heart and life to Him?
Never forget: Ultimately God is in control of this world, and no one will ever take His place or overturn His plans. And some day Christ will return, and when He does all evil will be destroyed and we will live in peace. Are you looking forward to that day?