best way to add custom space to template

I have a template that has two side bars both on the right hand side of the page. The total width of the two side bars is 415px. I need to ad an advertising space above these side bars that is the same width (as the two together).

I think it would be ideal add a block to contain this ad code. However, I am not sure how to get the block to show up in this space. I guess it would involve some custom code to pass some identifier to the template. Any suggestions on how to do this.

I could always hard code the ad code into the template, but I would prefer the flexibility of adding the code via a block.

Thanks for any suggestions.