Hi. Firstly, I need to commend you on your very active membership - it looks as if the average visitor visits 10+ pageviews at a time, so congratulations on making your site very "sticky". That's something I need more work on, myself.
Unfortunately, a sticky site does come with a very big downside. When you have such active visitors who are so enthralled with your site, it's very hard to convince them to click on the advertisements. In other words, there would be less banner blindness and more ad clicking if people DIDN'T want to stick around so long, and were semi-consciously looking for other places to go to. And the higher the click-thru rate, the more money you could get for the advertisements.
One of the best ad agencies I could recommend is Tribal Fusion. However, you might not yet have enough pageviews for them. I believe they have a 20,000/day minimum (I could be completely wrong on this), so you're halfway there.
Something that you may wish to try, however, especially because your site is so sticky, is to sell stickied threads within your forum. In other words, a company would write a thread advertising their product or service, and you would make it a sticky within a forum for one month for a fixed price. I think this would do very well on your site because forum members won't learn to ignore it as quickly as they would traditional banner advertising. Additionally, it's a good way to get some revenue coming in when you're still small.
Of course, I also highly suggest Google AdSense. For forums, the best ad spots are right beneath the first post in a thread, right after the last post in a thread but above the Quick Reply box, and right above all of the threads but below the navigation bar.