Does Yoga have any positive effect on aerobic capabilities or is it all just increasing flexibility of muscles and joints?

No... Yoga does not have any aerobics in it.  If you want to get an aerobic workout in addition to your yoga, I would go at least 3 or 4 times a week on a power walk, a run, or do some sort of aerobic activity in the gym.  Then you'll be getting a full workout for your body. 

Depends on what kind of Yoga you choose, Try my post over here :

http://yedda.com/questions/effective_Yoga_Pilates_diet_cardio_9518630815681/

Yoga is the ancient art of staying fit physically as well as mentally. All the different kinds of yoga have one common factor, which is the way you breathe. Breathing is important in yoga. You may find names like raja yoga, hatha yoga etc., but essentially any yoga is good. The following are wonderful yoga resources.

The Magic Of Finger Yoga

Complete Guide To Yogic Techniques For Happiness And Good Health

Yofa Healing Method Based On Yoga Meditation And Law Of Attraction

The 7 Minute Yoga Program For Stress Relief