ekool:
I realize that you own a few websites like this one. Firstly, let me commend you on that, it's a smart thing. Secondly, as I'm sure you know, being in the same business, advertising has a threshold that it must not cross.
When advertisements cross this threshold, they leave the realm of helpful and fiscally sound to pervasive and obtrusive. Only you, really, can decide where the threshold is for you, but from what I've seen, you're nearing it.
IMO, the green links are too much. It interrupts the reading of the forums. You are seeing such a large upswing in registrations to these forums because we have some very knowledgable, polite, and friendly people who submit content to them. An unwanted flood of excess advertising will disenfranchise a lot of users.
So when you cross the threshold, and the registration ebbs, you must ask yourself what's better: more users, or more ads?
Doesn't do much good to have a website crawling with advertisements if no one ever goes there.
Google is successful because it's simple and relatively ad-free (at least, that's where it started).
Best of luck to you, I know you'll make the right decision.
epoch.