[Simon]

I can easily call 9-10 different ads on a page via the single page call method without seeing much difference from having less ads on those pages.

Regarding having to redo pages. If you use templates you want the single page call in the header template so it's available to all your pages.

Also - if you're not using templates, do yourself a favor and stick some extra includes, either php or ssi, in the HEAD and at the beginning and end of each page's BODY tags. They can point to some empty files on your server so nothing gets inserted until you want to do something like add the OpenX single page call, or the Link Organizer tracking code, or Google analytics code to all of your pages. Then you can add that code to those formerly empty files and it'll get included from then on.

Most things I build now, except free sites and galleries, use some kind of template system, but using those 'empty' includes saved me a ton of time on earlier sites over the years.