It depends on ads placement, whether you'll place them just in the header, above/inside/at the end of the topic - just after the last post (if placed in viewtopic), in the footer. Ask Shawn, he should know about inside-topic placement About placement after the last post (1st placement) - didn't seem to perform very well - see my forums. So far I'm testing it, in a week or so probably will change placement a bit to see how new one will perform.
I'd be interested to know how that top banner works out for you. I put a small 5 link unit right there and in two days got zero clicks on it and it actaully decreased my total clicks for the rest of the pages!