I'm searching for relevant sites that I would like to link to me in the future (the site isn't built yet). If I were to buy banner spacew from them - does the fact that my banner links to my site count as a link or not?

Does it need to be more permanent to count as a useful link?

The site is relevant and I imagine it would be seen as an 'authority' site by the search engines.

Hope someone can set me straight here....