need help with editing multiple files in a directory

[bittna]

Hello,
  I have multiple files in a directory that need a line added at a
certain point in the file.  I wrote a lisp expression to do it, but I
have to load the file, then run the command on the buffer, the save
the file and I lose my place.  I used dired to do a find and replace
on all of the files I needed, but how do I run my lisp expression on
all of the files?

Also how do I loop through all the buffers using lisp and switch into
each buffer and do something, then move to the next buffer?

Thanks!!!