Firstly, welcome to the forum.
If the 800meg file is an MPG file and your burning it to a VCD or SVCD, then it will fit very comfortably on an 80min CDR. Just burn with Nero and you will see the capacity bar at the bottom will not exceed the 80min mark.
As the above has said, if its an AVI file, data file or collection of files, there is no way to get 800meg onto an 80min (700meg) CDR.