Use Pro-site to display ads on twentyten theme

Hello again,

I have created a network of sites for car dealers. Some of them have paid and others are free.

We have created a generic theme using twentyten child theme. This displays the cars for the particular deal on his own sub-site. We want to create one template theme from this which will handle both paid dealers (no ads) and free dealers (ads).

1. What code do we use to make this decisions on the template?

e.g. if supporter (show client header and sidebars) else (show ad headers and sidebars)

Thanks.