I am interested in writing my own internet ad server.

I want to serve billions of impressions with as little hardware possible.

Which server-side technologies are best suited for this task? I am asking about the relative overhead of serving my ad pages as either pages rendered by PHP, or Java, or .net, or coding Http responses directly in C and writing some multi-socket IO monster to serve requests (I assume this one wins, but if my assumption is wrong, that would actually be most interesting).

Obviously all the most efficient optimizations are done at the algorithm level, but I figure there has got to be some speed differences at the end of the day that makes one method of serving ads better than another. How much overhead does something like apache or IIS introduce? There's got to be a ton of extra junk in there I don't need.

At some point I guess this is more a question of which platform/language combo is best suited - please excuse the in-adroitly posed question, hopefully you understand what I am trying to get at.