There is nothing cut and dry about that. It all depends on what banners and pops you are using and what the place/s you are loading them from pays for them.
Most affiliate ads require some sort of action from your visitors - to click on an ad and then sign up for or buy something - and then you will get paid whatever commission there is for that particular action. There are some places that pay some amount of money for each so many impressions (usually so much per 1000 impressions), but even those normally require unique visitors - so if the same person sees the same ad 15 times during the one day (or whatever time frame they set), you will only get credit for the first time they see it, and most of the places that will pay for impressions require a high number of hits and don't usually like to have the ads on message boards because of low click through ratios on message boards.
Pop-Up ads will usually pay you more for impressions, but that is also at the price of upsetting your visitors (and they usually have to fully load to get credit, which often doesn't happen because of pop killers and fast-with-the-mouse visitors). Especially on message boards, pops can really hurt the activity on your boards because people don't like to have pops loading every time they go to read or post a message. SO... less people want to use the boards frequently, in the end you end up with less impressions than you would have with just a standard banner - IMO.
It will probably be a bit of trying different things before you figure out what works best with your site and/or forum visitors. Try to fit your ads with the interests of your visitors as much as possible too, that will help a lot. (Putting up ads for home decorating sites on your gaming site is probably not going to work too well, for example, but ads for sites that sell games might very well work out great.)
A bit of self-promotion here.. I have a page of links to affiliate programs I use myself at:
http://homepagebuilders.com/affiliate_programs.htm , you might find something that would work for you there.