Hi Chickadee,
Meg directed me to this post to try and sort out an answer for you. Circuit training is VERY effective for burning off fat, and if you lift HEAVY and do 2 sets per circuit, or do the circuit twice, it's even better. Just make sure that you lift for all your body parts. Cirucuit training doesn't necessarily get your heart rate any higher than non-circuit training, but since you don't get a rest period before moving onto another machine or set, it does have some cardio value. It's won't ever be a substitue for cardio, but part of the whole weightloss package.
If you are going to do weight training and cardio on the same day, I'd recommend doing your weights first, then moving to cardio. Additionally, don't spend more than an hour to an hour and a hlf on your workouts- beyond that it's counterproductive. You can set up a circuit that should take you 1/2 hr-45 minites and be very effective for a beginning weight lifter, then do 1/2 an hour of INTENSE cardio. Then get outta there! Only do a circuit 3 times a week, at most. When you are in the gym, you are breaking down nuscle tissue. It's during your resting phase, sleep and the next day) that the muscle gets built back up again but strong because of the good proteins and carbs you are eating.
If you want to do more cardio, that's fine, but try not to overload your lifting days.
Hope that helps
Mel