Ads within article?

In many blogs, you see advertisements nested within the text, like a block around which the text goes on.

Untill now, I’ve basically always used a three-column layout, to make room for the ads on both sides. But, I’d like to switch to a two-colomn layout, and replace one of the ads on the side by one within the text.

Is there a plugin, which allows me to embed some OpenX code within the article? Preferably where I can define two zones:

Zone a)
Appears when article exceeds 50 lines, halfway the content (so around line 25)

Zone b)
Appears when article exceeds 100 lines, at about 2/3rd of the content.

So longer articles could have two ads, while shorter ones have only one.

Of course, these linenumbers are arbitrary, it could also be a percentage or whatever.