well, the major physical benefits of yoga (and of course it depends on what type of yoga you are doing), is increasing flexibility, lengthening muscles, and stretching out your joints, and meditation. as you progress through yoga, you will definitely learn some serious strength training and balance (both incredibly important in core training).
i actually think that if you are taking a beginner's yoga class, you may consider doing your workout beforehand, because, like i said, one of the major benefits is flexibility, lengthening, and joint lubrication.
as you become more comfortable with yoga, stop taking the "standard" Hatha yoga gym class (a lot of focus on stretching), and look into more specific types of yoga.
Vinyasa- more strength training, mixed with stretching and concentration on breathing
Bikram- which will BURN calories, it's yoga done in a room heated to over 100f.
Anusara- yoga with props (blocks & straps) and alignment focus
Power- a more strenuous version of Vinyasa
Ashtanga- again,a more physically demanding form of yoga with quick stance changes and breathing techniques
Iyengar- like Ashtanga, but with a longer hold on the poses
HTH!!!