If you really want to build strength in your core, arms and upper body then don't do isolation excercises. Bicep curls and tricep pushdowns are a waste of time and you're just burning up energy which could be used for more functional excercises...Focus on your core, and concentrate on compound excercises such as shoulder press, rows, pull ups, push-ups, deadlifts and squats. If you're weak then start with light weights and slowly increase resistance and intensity.