my forum upgrade messed up and I'm told some files need 644 whilst others need 777.

Is there a list of files which need to be writable? I can't find it if there is

Update, I think it's these:

applications
datastore 
plugins
uploads
uploads/logs

 

Do I set all files and folders within these folders to 777? Just the files? just the folders? sub-directories too?

Those folders need to be writable. The specific permissions your site requires for each file and folder depends upon your host (or, more specifically, the technology your host is using).