I know you can limit  the number  of top-level  sites per content  
database, but can a site  collection span  mulitple content databases?
-nope

Is there a size  limit for a content database?
-not that i know of, if there was it would be a SQL/IIS limit


repository tool? as in replacing samba shares? im interested to know why, when your business decided that its file  shares werent good enough any more and then came to the conclusion that sharepoint was the next evolutionary step!

With a repository  of that size, you would probably be better off looking at a real Content Management System. SharePoint is great for what it does, but I don't think that it is designed to handle 80 TBs of data. An alternative approach might be to put the data  into a CMS, and provide an interface to it from SharePoint. I know that you can do this with Documentum, and probably with others as well.

It is highly unlikely that you are collaborating on 80 terabytes of
data.
So you may be better served if you buy SharePoint Portal to crawl the
historical data  in the 80 terabytes and only store the data used in
current collaboration efforts in SharePoint databases.
I hate to think of just moving that much data. You certainly would not
want to delete any in it's current location until you were convinced the
copy into the database  was rock solid. So now you have over 160
terrabytes.
Then you would want to back up the SharePoint installation before you
deleted the file  system copy.
What brand of storage are you using? I want to buy stock before you
start this project!

I would say that it would be an amazing accomplishment just to index that amount of data!