D4.6.8 to D6.10: Add Modules

Now you are ready to add the custom and D6 third-party modules you need. Coming from the world of D4.x, you need to be aware of something. The location for files and modules has changed.
Module Upload Location
Unlike D4, the D6 practice is to create a module directory either within your /sites/sitename directory or your /sites/all directory. Modules that will be shared across multiple sites can go in /sites/all directory. Modules that customize that specific site typically go in the /sites/sitename directory.
NOTE: If you are going to run multiple sites from your Drupal installation and you share the third-party modules across all the sites, then understand that upgrading the module will affect all sites using the module. In most cases, this will be okay. Just remember to check all your sites after the upgrade.
Files Location
Just like with modules, you want to create a files folder to hold your documents and images. If you choose to store docs and images in the /sites/all/files location, you will probably want to use a sub-directory or file naming process that helps you distinguish between the docs and images associated with specific sites. The path you use when calling the docs and images would be /sites/all/files/documents (assuming you call your doc directory documents). Or you would use /sites/sitename/files/documents for docs stored in the site specific directory.
NOTE: If you choose the /sites/sitename directory to store your files and images, please know that if you ever change the name of the directory (e.g., from default to examplesite.com), you will break all your links. Because, instead of looking for /sites/default/files/documents, the html needs to look for /sites/example.com/files/documents. Using the /sites/all approach is more efficient in the long run.
Module Dependencies
This isn't new, however, instead of simply having module dependencies, you also have PHP5.2 dependencies. PHP5 has been out for a while but if you have been hanging around in D4, it hasn't really been an issue. Now a lot of the cool stuff depends on PHP5.2. Depending on how many D4 legacy sites you have on your server, remember, it is the lowest common denominator that you need to worry about. D4, from my trials, doesn't run in PHP5.2. So until you get all your D4 sites into D6, you won't be able to add the cool modules that require PHP5.2. See my notes on D4.6.8 to D6.10: Upgrading to PHP5.2 to see what I am doing about juggling D4 and D6 sites.