IMO the ads are in the best locations possible and not too obtrusive (one at top, one in the middle, one towards the footer). Using 768x90.
A. Who clicks on ads? Forums have several sets of users. I won't get into the different kinds of forum site visitors but will focus on ad clickers. In my opinion these are the ad clickers:
1. Newbs who discover they need a certain product and see an ad for that product so they click on the ad.
2. Site visitors who are seeking specific product information then see a relevant ad for that product.
B. How many ads should you place in a forum? I think just two. Ads on top, below the navigation but before the actual content can get a decent click through rate. But ads below the content can also do well in a forum. Similar to the performance of affiliate ads. After raeding the content a site visitor may think , "Holy crap! My life/business/hobby is not the best unless I get one of those Acme Doohickys!" Ad Content with info about Acme Dohickys gets clicked.
C. Is this a money topic? A topic doesn't have to be about Mesothelioma to be money. It just has to have a first to second degree focus on products. Are there products involved? Downloads? Trials?
D. Expert forums do not sell Acme products Experts don't need products. They DIY or already have it. I said it before and I'll say it again: The only clicking experts do is on their keyboards, never on ads. Newbies buy products!
E. Inventory management Too many ad spots can cause crap ads to fill the excess inventory.