What is the exact gigabytes of 80 gigabytes?

Yes obviously... but double prefixes are officially prohibited by SI. A megagigabyte would actually be called a petabyte. 1000 kilobytes (kB) = 1 megabyte (MB) 1000 megabytes (MB) = 1 gigabyte (GB)...

There is exactly one gigabyte in a gigabyte.

It is an average size i'd say. Depends entirely what you're using it for. If you use it for games, or MP3's you might want a larger one. I'd recommend at least a 120GB one though.

1024 mb = 1 gb

There no such thing as a turbo gigabyte.