Don't worry about bulking up! Most ladies don't have the genetic ability to put on the masses of muscles that the boys have.

As for your question on your weight, my weight training (when i get around to doing it) uses reps of 8-12. Once I can do three sets of 12 reps on a certain weight, I know that I need to go heaver. Most arm excercises for me are more than 8 lbs, usually 15-25 except for triceps which are my weak spot and which are around 8.

Don't be afraid of going heavier with weights, you're bones will thank you for it in the future!

K.