I say put it at the top so it can be seen when visiting the site.
Hopefully people will click on it to generate you some kinda profit.
it could be a click based one but their usually annoying. They do ones where you get paid per 1000 views so noone has to click the adverts.
its a lot less pay for the advert, but its less intrusive and annoying. I was more thinking to put it at the bottom of the page above the footer, perhaps 2 next to each other the width of the page