What are your temps like with that setup?
As far as adding additional ventilation, I think your setup would become more efficient if you were to get ahold of some panda plastic and a tarp zipper. You've got a lot of air movement in there, but it is not as efficient as it could be. Ideally, you want a negative static pressure within your grow area. That allows good, clean air in, and keeps smelly air from escaping. Intakes at the bottom and exhaust at the top works in conjunction with the laws of thermaldynamics; hot air rises, cold air sinks. It is a lot easier to manage temps and airflow when you take advantage of this. As far as hooking up more lights/fans, I think you will find if you take my advice about the panda plastic and tarp zipper idea, you most likely won't have to add another fan. Additionally, if you use a heavy-duty extension cord with a quality surge protector, that should take care of your additional lights. Generally higher quality surge protectors have more useable outlets than lower quality ones.
There are a few threads on here dealing with home-made cooltube fixtures. This would also make a noticable difference in temps in your grow area, and result in higher yeilds due to the higher light penetration. Hope this helps.
