Well, I can help with the last question just a tad. We use phpadsnew to run most of the in-house banners on justusboys.com (not the spotlights though). We've never had any problems with it and its handled over 9 million adviews for us since we upgraded it back in the spring.

I think if you let it run on a powerful enough server it has quite a lot of muscle. We have a dedicated mysql box with dual xeons and 4GB of memory. But, that server is also handling our forum. Still it never breaks a load of more than 1.5 and its handling 864,751.33 queries per hour. So, give it a try on a decent box. I think its a great product.