I don't know the exact C# names, but it should be something like
str.Replace("(.{80})", "$1\r\n");
The idea is to grab 80 characters and save it in a group, then put it back in (I think "$1" is the right syntax) along with the "\r\n".
(Edit: The original regex had a +
in it, which you definitely don't want. That would completely eliminate everything except the last line and any leftover pieces--a decidedly suboptimal result.)
Note that this way, you will most likely split inside words, so it might look pretty ugly.
You should be looking more into word wrapping if this is indeed supposed to be readable text. A little googling turned up a couple of functions; or if this is a text box, you can just turn on the WordWrap property.
Also, check out the .Net page at regular-expressions.info. It's by far the best reference site for regexes that I know of. (Jan Goyvaerts is on SO, but nobody told me to say that.)